Re: [Xen-devel] [PATCH] x86-64: don't use xmalloc_array() for allocation of the (per-CPU) IDTs

[Keir Fraser <keir@xxxxxxx>]

On 13/10/2011 08:50, "Keir Fraser" <keir@xxxxxxx> wrote:

> On 13/10/2011 08:21, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
> 
>> The IDTs being exactly a page in size, using xmalloc() here is rather
>> inefficient, as this requires double the amount to be allocated (with
>> almost an entire page wasted). For hot plugged CPUs, this at once
>> eliminates one more non-order-zero runtime allocation.
>> 
>> For x86-32, however, the IDT is exactly half a page, so allocating a
>> full page seems wasteful here, so it continues to use xmalloc() as
>> before.
>> 
>> With most of the affected functions' bodies now being inside #ifdef-s,
>> it might be reasonable to split those parts out into subarch-specific
>> code...
> 
> Well, there is also opportunity for code merging. There is generally no
> reason for x86-64 to use alloc_domheap_pages where x86-32 uses
> alloc_xenheap_pages (e.g., for allocating per_cpu(gdt_table)) -- both
> architectures can use alloc_xenheap_pages in this case.
> 
> Also we might get rid of some further ifdef'ery if we added an alloc/free
> interface where *both* functions take a size parameter. We could then decide
> whether to use xmalloc or alloc_xenheap_pages dynamically based on that
> parameter. Knowing size on free would allow us to easily free such
> allocations too.
> 
> Finally, I'm not sure about more x86-64 code movement: I'd like to kill
> x86-32 entirely really.

Given our antipathy to the x86-32 hypervisor, and the fact that any
remaining users of it are unlikely to be running MP systems at all let alone
large MP systems, how about this cleanup patch?... (It looks quite confusing
as a patch, actually, but does the obvious thing).

 -- Keir

x86: Simplify smpboot_alloc by merging x86-{32,64} code as far as possible.

We still need one ifdef, as x86-32 does not have a compat_gdt_table.

On x86-32 there is 1/2-page wastage due to allocating a whole page for
the per-CPU IDT, however we expect very few users of the x86-32
hypervisor. Those that cannot move to the 64-bit hypervisor are likely
using old single-processor systems or new single-procesor netbooks. On
UP and small MP systems, the wastage is insignificant.

Signed-off-by: Keir Fraser <keir@xxxxxxx>
diff -r 1515484353c6 xen/arch/x86/smpboot.c
--- a/xen/arch/x86/smpboot.c    Thu Oct 13 10:09:28 2011 +0200
+++ b/xen/arch/x86/smpboot.c    Thu Oct 13 10:25:01 2011 +0100
@@ -640,21 +640,16 @@ static void cpu_smpboot_free(unsigned in
     unsigned int order;
 
     order = get_order_from_pages(NR_RESERVED_GDT_PAGES);
+    free_xenheap_pages(per_cpu(gdt_table, cpu), order);
+    per_cpu(gdt_table, cpu) = NULL;
+
 #ifdef __x86_64__
-    if ( per_cpu(compat_gdt_table, cpu) )
-        free_domheap_pages(virt_to_page(per_cpu(gdt_table, cpu)), order);
-    if ( per_cpu(gdt_table, cpu) )
-        free_domheap_pages(virt_to_page(per_cpu(compat_gdt_table, cpu)),
-                           order);
+    free_xenheap_pages(per_cpu(compat_gdt_table, cpu), order);
     per_cpu(compat_gdt_table, cpu) = NULL;
-    order = get_order_from_bytes(IDT_ENTRIES * sizeof(**idt_tables));
-    if ( idt_tables[cpu] )
-        free_domheap_pages(virt_to_page(idt_tables[cpu]), order);
-#else
-    free_xenheap_pages(per_cpu(gdt_table, cpu), order);
-    xfree(idt_tables[cpu]);
 #endif
-    per_cpu(gdt_table, cpu) = NULL;
+
+    order = get_order_from_bytes(IDT_ENTRIES * sizeof(idt_entry_t));
+    free_xenheap_pages(idt_tables[cpu], order);
     idt_tables[cpu] = NULL;
 
     if ( stack_base[cpu] != NULL )
@@ -669,9 +664,6 @@ static int cpu_smpboot_alloc(unsigned in
 {
     unsigned int order;
     struct desc_struct *gdt;
-#ifdef __x86_64__
-    struct page_info *page;
-#endif
 
     stack_base[cpu] = alloc_xenheap_pages(STACK_ORDER, 0);
     if ( stack_base[cpu] == NULL )
@@ -679,41 +671,28 @@ static int cpu_smpboot_alloc(unsigned in
     memguard_guard_stack(stack_base[cpu]);
 
     order = get_order_from_pages(NR_RESERVED_GDT_PAGES);
-#ifdef __x86_64__
-    page = alloc_domheap_pages(NULL, order,
-                               MEMF_node(cpu_to_node(cpu)));
-    if ( !page )
+    per_cpu(gdt_table, cpu) = gdt =
+        alloc_xenheap_pages(order, MEMF_node(cpu_to_node(cpu)));
+    if ( gdt == NULL )
         goto oom;
-    per_cpu(compat_gdt_table, cpu) = gdt = page_to_virt(page);
-    memcpy(gdt, boot_cpu_compat_gdt_table,
-           NR_RESERVED_GDT_PAGES * PAGE_SIZE);
-    gdt[PER_CPU_GDT_ENTRY - FIRST_RESERVED_GDT_ENTRY].a = cpu;
-    page = alloc_domheap_pages(NULL, order,
-                               MEMF_node(cpu_to_node(cpu)));
-    if ( !page )
-        goto oom;
-    per_cpu(gdt_table, cpu) = gdt = page_to_virt(page);
-    order = get_order_from_bytes(IDT_ENTRIES * sizeof(**idt_tables));
-    page = alloc_domheap_pages(NULL, order,
-                               MEMF_node(cpu_to_node(cpu)));
-    if ( !page )
-        goto oom;
-    idt_tables[cpu] = page_to_virt(page);
-#else
-    per_cpu(gdt_table, cpu) = gdt = alloc_xenheap_pages(order, 0);
-    if ( !gdt )
-        goto oom;
-    idt_tables[cpu] = xmalloc_array(idt_entry_t, IDT_ENTRIES);
-    if ( idt_tables[cpu] == NULL )
-        goto oom;
-#endif
-    memcpy(gdt, boot_cpu_gdt_table,
-           NR_RESERVED_GDT_PAGES * PAGE_SIZE);
+    memcpy(gdt, boot_cpu_gdt_table, NR_RESERVED_GDT_PAGES * PAGE_SIZE);
     BUILD_BUG_ON(NR_CPUS > 0x10000);
     gdt[PER_CPU_GDT_ENTRY - FIRST_RESERVED_GDT_ENTRY].a = cpu;
 
-    memcpy(idt_tables[cpu], idt_table,
-           IDT_ENTRIES*sizeof(idt_entry_t));
+#ifdef __x86_64__
+    per_cpu(compat_gdt_table, cpu) = gdt =
+        alloc_xenheap_pages(order, MEMF_node(cpu_to_node(cpu)));
+    if ( gdt == NULL )
+        goto oom;
+    memcpy(gdt, boot_cpu_compat_gdt_table, NR_RESERVED_GDT_PAGES *
PAGE_SIZE);
+    gdt[PER_CPU_GDT_ENTRY - FIRST_RESERVED_GDT_ENTRY].a = cpu;
+#endif
+
+    order = get_order_from_bytes(IDT_ENTRIES * sizeof(idt_entry_t));
+    idt_tables[cpu] = alloc_xenheap_pages(order,
MEMF_node(cpu_to_node(cpu)));
+    if ( idt_tables[cpu] == NULL )
+        goto oom;
+    memcpy(idt_tables[cpu], idt_table, IDT_ENTRIES * sizeof(idt_entry_t));
 
     return 0;
 

> Anyway, despite all this...
> 
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Acked-by: Keir Fraser <keir@xxxxxxx>
> 
>> --- a/xen/arch/x86/smpboot.c
>> +++ b/xen/arch/x86/smpboot.c
>> @@ -639,9 +639,6 @@ static void cpu_smpboot_free(unsigned in
>>  {
>>      unsigned int order;
>>  
>> -    xfree(idt_tables[cpu]);
>> -    idt_tables[cpu] = NULL;
>> -
>>      order = get_order_from_pages(NR_RESERVED_GDT_PAGES);
>>  #ifdef __x86_64__
>>      if ( per_cpu(compat_gdt_table, cpu) )
>> @@ -650,10 +647,15 @@ static void cpu_smpboot_free(unsigned in
>>          free_domheap_pages(virt_to_page(per_cpu(compat_gdt_table, cpu)),
>>                             order);
>>      per_cpu(compat_gdt_table, cpu) = NULL;
>> +    order = get_order_from_bytes(IDT_ENTRIES * sizeof(**idt_tables));
>> +    if ( idt_tables[cpu] )
>> +        free_domheap_pages(virt_to_page(idt_tables[cpu]), order);
>>  #else
>>      free_xenheap_pages(per_cpu(gdt_table, cpu), order);
>> +    xfree(idt_tables[cpu]);
>>  #endif
>>      per_cpu(gdt_table, cpu) = NULL;
>> +    idt_tables[cpu] = NULL;
>>  
>>      if ( stack_base[cpu] != NULL )
>>      {
>> @@ -691,19 +693,25 @@ static int cpu_smpboot_alloc(unsigned in
>>      if ( !page )
>>          goto oom;
>>      per_cpu(gdt_table, cpu) = gdt = page_to_virt(page);
>> +    order = get_order_from_bytes(IDT_ENTRIES * sizeof(**idt_tables));
>> +    page = alloc_domheap_pages(NULL, order,
>> +                               MEMF_node(cpu_to_node(cpu)));
>> +    if ( !page )
>> +        goto oom;
>> +    idt_tables[cpu] = page_to_virt(page);
>>  #else
>>      per_cpu(gdt_table, cpu) = gdt = alloc_xenheap_pages(order, 0);
>>      if ( !gdt )
>>          goto oom;
>> +    idt_tables[cpu] = xmalloc_array(idt_entry_t, IDT_ENTRIES);
>> +    if ( idt_tables[cpu] == NULL )
>> +        goto oom;
>>  #endif
>>      memcpy(gdt, boot_cpu_gdt_table,
>>             NR_RESERVED_GDT_PAGES * PAGE_SIZE);
>>      BUILD_BUG_ON(NR_CPUS > 0x10000);
>>      gdt[PER_CPU_GDT_ENTRY - FIRST_RESERVED_GDT_ENTRY].a = cpu;
>>  
>> -    idt_tables[cpu] = xmalloc_array(idt_entry_t, IDT_ENTRIES);
>> -    if ( idt_tables[cpu] == NULL )
>> -        goto oom;
>>      memcpy(idt_tables[cpu], idt_table,
>>             IDT_ENTRIES*sizeof(idt_entry_t));
>>  
>> 
>> 
>> 
>> 
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>> http://lists.xensource.com/xen-devel
> 
> 



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