Re: [Xen-devel] [PATCH] libx86: Introduce wrappers for extracting XCR0/XSS from a cpuid policy

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 23/05/2019 12:52, Jan Beulich wrote:
>>>> On 23.05.19 at 12:27, <andrew.cooper3@citrix.com> wrote:
>> --- a/xen/arch/x86/xstate.c
>> +++ b/xen/arch/x86/xstate.c
>> @@ -660,9 +660,7 @@ static bool valid_xcr0(u64 xcr0)
>>  int validate_xstate(const struct domain *d, uint64_t xcr0, uint64_t xcr0_accum,
>>                      const struct xsave_hdr *hdr)
>>  {
>> -    const struct cpuid_policy *cp = d->arch.cpuid;
>> -    uint64_t xcr0_max =
>> -        ((uint64_t)cp->xstate.xcr0_high << 32) | cp->xstate.xcr0_low;
>> +    uint64_t xcr0_max = cpuid_policy_xcr0(d->arch.cpuid);
>>      unsigned int i;
>>  
>>      if ( (hdr->xstate_bv & ~xcr0_accum) ||
>> @@ -686,9 +684,7 @@ int validate_xstate(const struct domain *d, uint64_t xcr0, uint64_t xcr0_accum,
>>  int handle_xsetbv(u32 index, u64 new_bv)
>>  {
>>      struct vcpu *curr = current;
>> -    const struct cpuid_policy *cp = curr->domain->arch.cpuid;
>> -    uint64_t xcr0_max =
>> -        ((uint64_t)cp->xstate.xcr0_high << 32) | cp->xstate.xcr0_low;
>> +    uint64_t xcr0_max = cpuid_policy_xcr0(curr->domain->arch.cpuid);
> In both cases the variables are more appropriately named than
> the new helper. While I agree it's slightly more typing, did you
> consider calling it cpuid_policy_xcr0_max()?

Fine.

>
>> --- a/xen/include/xen/lib/x86/cpuid.h
>> +++ b/xen/include/xen/lib/x86/cpuid.h
>> @@ -308,6 +308,18 @@ static inline void cpuid_featureset_to_policy(
>>      p->feat._7a1  = fs[FEATURESET_7a1];
>>  }
>>  
>> +static inline uint64_t cpuid_policy_xcr0(const struct cpuid_policy *p)
>> +{
>> +    return ((uint64_t)p->xstate.xcr0_high << 32) | p->xstate.xcr0_low;
>> +}
>> +
>> +static inline uint64_t cpuid_policy_xstates(const struct cpuid_policy *p)
>> +{
>> +    uint64_t val = p->xstate.xcr0_high | p->xstate.xss_high;
>> +
>> +    return (val << 32) | p->xstate.xcr0_low | p->xstate.xss_low;
>> +}
> How about also having cpuid_policy_xss() (or cpuid_policy_xss_max())
> and then simply making cpuid_policy_xstates() combine the two
> results?

I started with that, but the resulting code was a little awkward to
read, and the asm generation was a little worse due to promoting
everything first.

I don't think we need cpuid_policy_xss{,_max}() until we actually
implement something for guests (most likely CET at this rate).

>
> Anyway, as I can also live with things as they are, with or without
> either of the suggested changes
> Reviewed-by: Jan Beulich <jbeulich@suse.com>

Thanks (although I'm still happy to play around with naming).

~Andrew

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