Re: [Xen-devel] [PATCH 3 of 3] KEXEC: Allocate crash structures in low memory

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/12/11 09:06, Jan Beulich wrote:
>>>> On 22.12.11 at 18:36, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 64bit Xen with 32bit dom0 and crashkernel, xmalloc'ing items such
>> as the CPU crash notes will go into the xenheap, which tends to be in
>> upper memory.  This causes problems on machines with more than 64GB
>> (or 4GB if no PAE support) of ram as the crashkernel physically cant
>> access the crash notes.
> What use is a crash dump taken with a 32-bit kernel on a machine
> with more than 64G (or more than 4G is the crash kernel doesn't
> support PAE)?

Very little use at all, which is the reason for this change.  The
correct solution is indeed to use a 64bit dom0 kernel, but while
politics is preventing that from happening, another solution has to be
found.  I doubt that XenServer is the only product which would benifit,
which is why the changes are presented for upstreaming.

>> The solution is to force Xen to allocate certain structures in lower
>> memory.  This is achieved by introducing two new command line
>> parameters; low_crashinfo and crashinfo_maxaddr.  Because of the
>> potential impact on 32bit PV guests, and that this problem does not
>> exist for 64bit dom0 on 64bit Xen, this new functionality defaults to
>> the codebase's previous behavior, requiring the user to explicitly
>> add extra command line parameters to change the behavior.
>>
>> This patch consists of 3 logically distinct but closely related
>> changes.
>>  1) Add the two new command line parameters.
> Do you really need both? Just being able to set the max address
> would seem sufficient...

The plan, as explained in the comments, is for two (or perhaps more)
levels of low allocation.  "min" means "allocate enough stuff low for
the crash kernel to grab the Xen ring buffer" where "all" will include
extra things such as the page tables so the crash kernel can generate
full stack traces given the PT_STATUS notes.  The differentiation is
because "min" should not impact very much at all on other memory
restricted things such as 32bit PV guests, whereas "all" certainly will.

>>  2) Change crash note allocation to use lower memory when instructed.
>>  3) Change the conring buffer to use lower memory when instructed.
> Why does this one need moving, but none of the other Xen data
> structures? If anything, I would have expected you to limit the Xen
> heap altogether, so that automatically all allocations get done in a
> way so that at least Xen's data structures are available in the
> (truncated) dump.

This is part of the "min" option which is trying to have the least
possible impact.  The idea is to have this in low memory, use the
"console_to_ring" boot option to copy dom0 dmesg into conring, and pass
its physical address and size in a crash note, so that the crash kernel
environment grab it all.

>> There result is that the crash notes and console ring will be placed
>> in lower memory so useful information can be recovered in the case of
>> a crash.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>
>> diff -r 3da37c68284f -r 23c31d59ffb1 xen/arch/x86/setup.c
>> --- a/xen/arch/x86/setup.c
>> +++ b/xen/arch/x86/setup.c
>> @@ -586,6 +586,10 @@ void __init __start_xen(unsigned long mb
>>      }
>>      cmdline_parse(cmdline);
>>  
>> +    /* Must be after command line argument parsing and before 
>> +     * allocing any xenheap structures wanted in lower memory. */
>> +    kexec_early_calculations();
>> +
> But does it really need to be *this* early?

Possibly not, but it needs to be before console_preirq_init() which is
not much later.

>>      parse_video_info();
>>  
>>      set_current((struct vcpu *)0xfffff000); /* debug sanity */
>> diff -r 3da37c68284f -r 23c31d59ffb1 xen/common/kexec.c
>> --- a/xen/common/kexec.c
>> +++ b/xen/common/kexec.c
>> @@ -36,7 +36,9 @@ bool_t kexecing = FALSE;
>>  typedef struct { Elf_Note * start; size_t size; } crash_note_range_t;
>>  static crash_note_range_t * crash_notes;
>>  
>> -/* Lock to prevent race conditions when allocating the crash note buffers. 
>> */
>> +/* Lock to prevent race conditions when allocating the crash note buffers.
>> + * It also serves to protect calls to alloc_from_crash_heap when allocating
>> + * crash note buffers in lower memory. */
>>  static DEFINE_SPINLOCK(crash_notes_lock);
>>  
>>  static Elf_Note *xen_crash_note;
>> @@ -70,6 +72,23 @@ static struct {
>>      (_d_)->start = (_s_)->start; \
>>  } while (0)
>>  
>> +/* Low crashinfo mode.  Start as INVALID so serveral codepaths can set up
>> + * defaults without needing to know the state of the others. */
>> +enum low_crashinfo low_crashinfo_mode = LOW_CRASHINFO_INVALID;
>> +
>> +/* This value is only considered if low_crash_mode is set to MIN or ALL, so
>> + * setting a default here is safe. Default to 4GB.  This is because the 
>> current
>> + * KEXEC_CMD_get_range compat hypercall trucates 64bit pointers to 32 bits. 
>> The
>> + * typical usecase for crashinfo_maxaddr will be for 64bit Xen with 32bit 
>> dom0 
>> + * and 32bit crash kernel. */
>> +static paddr_t crashinfo_maxaddr = 4ULL << 30;
> __initdata

Good point

>> +
>> +/* = log base 2 of crashinfo_maxaddr after checking for sanity. */
>> +paddr_t crashinfo_maxaddr_bits = 0;
>> +
>> +/* Pointers to keep track of the crash heap region. */
>> +static void *crash_heap_current = NULL, *crash_heap_end = NULL;
>> +
>>  /*
>>   * Parse command lines in the format
>>   *
>> @@ -148,6 +167,58 @@ static void __init parse_crashkernel(con
>>  }
>>  custom_param("crashkernel", parse_crashkernel);
>>  
>> +/* Parse command lines in the format:
>> + *
>> + *   low_crashinfo=[none,min,all]
>> + * 
>> + * - none disables the low allocation of crash info.
>> + * - min will allocate enough low information for the crash kernel to be 
>> able
>> + *       to extract the hypervisor and dom0 message ring buffers.
>> + * - all will allocate additional structures such as domain and vcpu structs
>> + *       low so the crash kernel can perform an extended analysis of state.
>> + */
>> +static void __init parse_low_crashinfo(const char * str)
>> +{
>> +
>> +    if ( !strlen(str) )
>> +        /* default to min if user just specifies "low_crashinfo" */
>> +        low_crashinfo_mode = LOW_CRASHINFO_MIN;
>> +    else if ( !strcmp(str, "none" ) )
>> +        low_crashinfo_mode = LOW_CRASHINFO_NONE;
>> +    else if ( !strcmp(str, "min" ) )
>> +        low_crashinfo_mode = LOW_CRASHINFO_MIN;
>> +    else if ( !strcmp(str, "all" ) )
>> +        low_crashinfo_mode = LOW_CRASHINFO_ALL;
>> +    else
>> +    {
>> +        printk("Unknown low_crashinfo parameter '%s'.  Defaulting to 
>> min.\n", str);
>> +        low_crashinfo_mode = LOW_CRASHINFO_MIN;
>> +    }
>> +}
>> +custom_param("low_crashinfo", parse_low_crashinfo);
>> +
>> +/* Parse command lines in the format:
>> + * 
>> + *   crashinfo_maxaddr=<addr>
>> + *
>> + * <addr> will be rounded down to the nearest power of two.  Defaults to 64G
>> + */
>> +static void __init parse_crashinfo_maxaddr(const char * str)
>> +{
>> +    u64 addr;
>> +
>> +    /* if low_crashinfo_mode is unset, default to min. */
>> +    if ( low_crashinfo_mode == LOW_CRASHINFO_INVALID )
>> +        low_crashinfo_mode = LOW_CRASHINFO_MIN;
>> +
>> +    if ( (addr = parse_size_and_unit(str, NULL)) )
>> +        crashinfo_maxaddr = addr;
>> +    else
>> +        printk("Unable to parse crashinfo_maxaddr. Defaulting to %p\n",
>> +               (void*)crashinfo_maxaddr);
>> +}
>> +custom_param("crashinfo_maxaddr", parse_crashinfo_maxaddr);
>> +
>>  void __init set_kexec_crash_area_size(u64 system_ram)
>>  {
>>      unsigned int idx;
>> @@ -306,6 +377,20 @@ static size_t sizeof_cpu_notes(const uns
>>      return bytes;
>>  }
>>  
>> +/* Allocate size_t bytes of space from the previously allocated
>> + * crash heap if the user has requested that crash notes be allocated
>> + * in lower memory.  There is currently no case where the crash notes
>> + * should be free()'d. */
>> +static void * alloc_from_crash_heap(const size_t bytes)
>> +{
>> +    void * ret;
>> +    if ( crash_heap_current + bytes > crash_heap_end )
>> +        return NULL;
>> +    ret = (void*)crash_heap_current;
>> +    crash_heap_current += bytes;
>> +    return ret;
>> +}
>> +
>>  /* Allocate a crash note buffer for a newly onlined cpu. */
>>  static int kexec_init_cpu_notes(const unsigned long cpu)
>>  {
>> @@ -320,7 +405,10 @@ static int kexec_init_cpu_notes(const un
>>          return ret;
>>  
>>      nr_bytes = sizeof_cpu_notes(cpu);
>> -    note = xmalloc_bytes(nr_bytes);
>> +
>> +    /* If we dont care about the position of allocation, malloc. */
>> +    if ( low_crashinfo_mode == LOW_CRASHINFO_NONE )
>> +        note = xmalloc_bytes(nr_bytes);
>>  
>>      /* Protect the write into crash_notes[] with a spinlock, as this 
>> function
>>       * is on a hotplug path and a hypercall path. */
>> @@ -338,6 +426,11 @@ static int kexec_init_cpu_notes(const un
>>      }
>>      else
>>      {
>> +        /* If we care about memory possition, alloc from the crash heap,
>> +         * also protected by the crash_notes_lock. */
>> +        if ( low_crashinfo_mode > LOW_CRASHINFO_NONE )
>> +            note = alloc_from_crash_heap(nr_bytes);
>> +
>>          crash_notes[cpu].start = note;
>>          crash_notes[cpu].size = nr_bytes;
>>          spin_unlock(&crash_notes_lock);
>> @@ -397,6 +490,24 @@ static struct notifier_block cpu_nfb = {
>>      .notifier_call = cpu_callback
>>  };
>>  
>> +void __init kexec_early_calculations(void)
>> +{
>> +    /* If low_crashinfo_mode is still INVALID, neither "low_crashinfo" nor
>> +     * "crashinfo_maxaddr" have been specified on the command line, so
>> +     * explicitly set to NONE. */
>> +    if ( low_crashinfo_mode == LOW_CRASHINFO_INVALID )
>> +        low_crashinfo_mode = LOW_CRASHINFO_NONE;
>> +
>> +    crashinfo_maxaddr_bits = 0;
>> +    if ( low_crashinfo_mode > LOW_CRASHINFO_NONE )
>> +    {
>> +        paddr_t tmp = crashinfo_maxaddr;
>> +
>> +        while ( tmp >>= 1 )
>> +            crashinfo_maxaddr_bits++;
> fls() or some of its cousins

Ok

>> +    }
>> +}
>> +
>>  static int __init kexec_init(void)
>>  {
>>      void *cpu = (void *)(unsigned long)smp_processor_id();
>> @@ -407,6 +518,29 @@ static int __init kexec_init(void)
>>  
>>      register_keyhandler('C', &crashdump_trigger_keyhandler);
>>  
>> +    if ( low_crashinfo_mode > LOW_CRASHINFO_NONE )
>> +    {
>> +        size_t crash_heap_size;
>> +
>> +        /* This calculation is safe even if the machine is booted in 
>> +         * uniprocessor mode. */
>> +        crash_heap_size = sizeof_cpu_notes(0) +
>> +            sizeof_cpu_notes(1) * (nr_cpu_ids - 1);
>> +        crash_heap_size = PAGE_ALIGN(crash_heap_size);
>> +
>> +        crash_heap_current = alloc_xenheap_pages(
>> +            get_order_from_bytes(crash_heap_size),
>> +            MEMF_bits(crashinfo_maxaddr_bits) );
>> +
>> +        if ( crash_heap_current )
>> +            crash_heap_end = crash_heap_current + crash_heap_size;
>> +        else
>> +            return -ENOMEM;
> Constructs like this generally look bogus to me, as
>
>         if ( !crash_heap_current )
>             return -ENOMEM;
>         ...
>
> is shorter and (to me at least) more clear.

True - for a long time I was debating whether to return ENOMEM at this
point at not, so I clearly didn't consider clarity when deciding to add
it in.  I will fix this.

>> +    }
>> +
>> +    /* crash_notes may be allocated anywhere Xen can reach in memory.
>> +       Only the individual CPU crash notes themselves must be allocated
>> +       in lower memory if requested. */
>>      crash_notes = xmalloc_array(crash_note_range_t, nr_cpu_ids);
>>      if ( ! crash_notes )
>>          return -ENOMEM;
>> diff -r 3da37c68284f -r 23c31d59ffb1 xen/drivers/char/console.c
>> --- a/xen/drivers/char/console.c
>> +++ b/xen/drivers/char/console.c
>> @@ -584,6 +584,7 @@ void __init console_init_postirq(void)
>>  {
>>      char *ring;
>>      unsigned int i, order;
>> +    u64 memflags;
>>  
>>      serial_init_postirq();
>>  
>> @@ -591,7 +592,8 @@ void __init console_init_postirq(void)
>>          opt_conring_size = num_present_cpus() << (9 + xenlog_lower_thresh);
>>  
>>      order = get_order_from_bytes(max(opt_conring_size, conring_size));
>> -    while ( (ring = alloc_xenheap_pages(order, 0)) == NULL )
>> +    memflags = low_crashinfo_mode > LOW_CRASHINFO_NONE ? 
>> MEMF_bits(crashinfo_maxaddr_bits) : 0;
>> +    while ( (ring = alloc_xenheap_pages(order, memflags)) == NULL )
>>      {
>>          BUG_ON(order == 0);
>>          order--;
>> diff -r 3da37c68284f -r 23c31d59ffb1 xen/include/xen/kexec.h
>> --- a/xen/include/xen/kexec.h
>> +++ b/xen/include/xen/kexec.h
>> @@ -25,6 +25,19 @@ void set_kexec_crash_area_size(u64 syste
>>  #define KEXEC_IMAGE_CRASH_BASE   2
>>  #define KEXEC_IMAGE_NR           4
>>  
>> +enum low_crashinfo {
>> +    LOW_CRASHINFO_INVALID = 0,
>> +    LOW_CRASHINFO_NONE = 1,
>> +    LOW_CRASHINFO_MIN = 2,
>> +    LOW_CRASHINFO_ALL = 3
>> +};
>> +
>> +/* Low crashinfo mode.  Start as INVALID so serveral codepaths can set up
>> + * defaults without needing to know the state of the others. */
>> +extern enum low_crashinfo low_crashinfo_mode;
>> +extern paddr_t crashinfo_maxaddr_bits;
>> +void kexec_early_calculations(void);
>> +
>>  int machine_kexec_load(int type, int slot, xen_kexec_image_t *image);
>>  void machine_kexec_unload(int type, int slot, xen_kexec_image_t *image);
>>  void machine_kexec_reserved(xen_kexec_reserve_t *reservation);
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxxxxxxxx 
>> http://lists.xensource.com/xen-devel 
>

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel