Re: [Xen-devel] [PATCH v6 09/12] xen: add runtime parameter access support to hypfs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.03.2020 16:07, Jürgen Groß wrote:
> On 04.03.20 12:32, Jan Beulich wrote:
>> On 26.02.2020 13:47, Juergen Gross wrote:
>>> +static void update_ept_param_append(const char *str, int val)
>>> +{
>>> +    char *pos = opt_ept_setting + strlen(opt_ept_setting);
>>> +
>>> +    snprintf(pos, sizeof(opt_ept_setting) - (pos - opt_ept_setting),
>>> +             ",%s=%d", str, val);
>>> +}
>>> +
>>> +static void update_ept_param(void)
>>> +{
>>> +    snprintf(opt_ept_setting, sizeof(opt_ept_setting), "pml=%d", 
>>> opt_ept_pml);
>>> +    if ( opt_ept_ad >= 0 )
>>> +        update_ept_param_append("ad", opt_ept_ad);
>>
>> This won't correctly reflect reality: If you look at
>> vmx_init_vmcs_config(), even a negative value means "true" here,
>> unless on a specific Atom model. I think init_ept_param() wants
>> to have that erratum workaround logic moved there, such that
>> you can then assme the value to be non-negative here.
> 
> But isn't not mentioning it in the -1 case correct? -1 means: do the
> correct thing on the current hardware.

Well, I think the output here should represent effective settings,
and a sub-item should be suppressed only if a setting has no effect
at all in the current setup, like ...

>>> +    if ( opt_ept_exec_sp >= 0 )
>>> +        update_ept_param_append("exec-sp", opt_ept_exec_sp);
>>
>> I agree for this one - if the value is still -1, it has neither
>> been set nor is its value of any interest.

... here.

>>> --- a/xen/common/grant_table.c
>>> +++ b/xen/common/grant_table.c
>>> @@ -85,8 +85,10 @@ struct grant_table {
>>>       struct grant_table_arch arch;
>>>   };
>>>   
>>> -static int parse_gnttab_limit(const char *param, const char *arg,
>>> -                              unsigned int *valp)
>>> +#define GRANT_CUSTOM_VAL_SZ  12
>>> +
>>> +static int parse_gnttab_limit(const char *arg, unsigned int *valp,
>>> +                              char *parval)
>>>   {
>>>       const char *e;
>>>       unsigned long val;
>>> @@ -99,28 +101,47 @@ static int parse_gnttab_limit(const char *param, const 
>>> char *arg,
>>>           return -ERANGE;
>>>   
>>>       *valp = val;
>>> +    snprintf(parval, GRANT_CUSTOM_VAL_SZ, "%lu", val);
>>>   
>>>       return 0;
>>>   }
>>>   
>>>   unsigned int __read_mostly opt_max_grant_frames = 64;
>>> +static char __read_mostly opt_max_grant_frames_val[GRANT_CUSTOM_VAL_SZ];
>>> +
>>> +static void __init gnttab_max_frames_init(struct param_hypfs *par)
>>> +{
>>> +    custom_runtime_set_var(par, opt_max_grant_frames_val);
>>
>> You still use a custom string buffer here. Can this "set-var"
>> operation record that the variable (for presentation purposes)
>> is simply of UINT type, handing a pointer to the actual
>> variable?
> 
> No, this would result in the need to set a custom parameter via a
> binary value passed in from user land. So I'd need to convert this
> binary into a string to be parseable by the custom function.

Hmm, not very fortunate, but I can see what you're saying.

>>> --- a/xen/common/hypfs.c
>>> +++ b/xen/common/hypfs.c
>>> @@ -10,6 +10,7 @@
>>>   #include <xen/hypercall.h>
>>>   #include <xen/hypfs.h>
>>>   #include <xen/lib.h>
>>> +#include <xen/param.h>
>>>   #include <xen/rwlock.h>
>>>   #include <public/hypfs.h>
>>>   
>>> @@ -281,6 +282,33 @@ int hypfs_write_bool(struct hypfs_entry_leaf *leaf,
>>>       return 0;
>>>   }
>>>   
>>> +int hypfs_write_custom(struct hypfs_entry_leaf *leaf,
>>> +                       XEN_GUEST_HANDLE_PARAM(void) uaddr, unsigned long 
>>> ulen)
>>> +{
>>> +    struct param_hypfs *p;
>>> +    char *buf;
>>> +    int ret;
>>> +
>>> +    buf = xzalloc_array(char, ulen);
>>> +    if ( !buf )
>>> +        return -ENOMEM;
>>> +
>>> +    ret = -EFAULT;
>>> +    if ( copy_from_guest(buf, uaddr, ulen) )
>>> +        goto out;
>>> +
>>> +    ret = -EDOM;
>>> +    if ( !memchr(buf, 0, ulen) )
>>
>> Once again " != buf + ulen - 1"? (EDOM also looks like an odd
>> error code to use in this case, but I guess there's no really
>> good one.)
> 
> " != buf + ulen - 1" is a logical choice with the change of patch 4.

I'm afraid I don't understand. You want to parse a string here.
The caller should tell you what the string length is (including
the nul again), not what its buffer size may be.

>>> @@ -79,41 +88,94 @@ extern const struct kernel_param __param_start[], 
>>> __param_end[];
>>>             .type = OPT_IGNORE }
>>>   
>>>   #define __rtparam         __param(__dataparam)
>>> +#define __paramfs         static __paramhypfs \
>>> +    __attribute__((__aligned__(sizeof(void *)))) struct param_hypfs
>>>   
>>> -#define custom_runtime_only_param(_name, _var) \
>>> +#define custom_runtime_set_var(parfs, var) \
>>> +    { \
>>> +        (parfs)->hypfs.write_ptr = &(var); \
>>> +        (parfs)->hypfs.e.size = sizeof(var); \
>>
>> All users of this use char[]. Why sizeof() rather than strlen(),
> 
> That is the maximum string length. Otherwise I wouldn't know I am
> allowed to replace e.g. "on" by "noxpti".

As said elsewhere - if e.size is the buffer size, then the
reading function wants adjusting, and it needs to be clarified
how buffer size and payload size can be told apart for BLOBs.

Jan

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