Re: [PATCH v2 3/4] x86/cpu-policy: Simplify recalculate_xstate()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 02/05/2024 1:39 pm, Jan Beulich wrote:
> On 29.04.2024 20:28, Andrew Cooper wrote:
>> Make use of the new xstate_uncompressed_size() helper rather than maintaining
>> the running calculation while accumulating feature components.
> xstate_uncompressed_size() isn't really a new function, but the re-work of
> an earlier one. That, aiui, could have been used here, too, just that it
> would have been inefficient to do so. IOW perhaps drop "the new"?

Ok.

>
>> The rest of the CPUID data can come direct from the raw cpu policy.  All
>> per-component data form an ABI through the behaviour of the X{SAVE,RSTOR}*
>> instructions.
>>
>> Use for_each_set_bit() rather than opencoding a slightly awkward version of
>> it.  Mask the attributes in ecx down based on the visible features.  This
>> isn't actually necessary for any components or attributes defined at the time
>> of writing (up to AMX), but is added out of an abundance of caution.
> As to this, ...
>
>> @@ -206,61 +205,47 @@ static void recalculate_xstate(struct cpu_policy *p)
>>          return;
>>  
>>      if ( p->basic.avx )
>> -    {
>>          xstates |= X86_XCR0_YMM;
>> -        xstate_size = max(xstate_size,
>> -                          xstate_offsets[X86_XCR0_YMM_POS] +
>> -                          xstate_sizes[X86_XCR0_YMM_POS]);
>> -    }
>>  
>>      if ( p->feat.mpx )
>> -    {
>>          xstates |= X86_XCR0_BNDREGS | X86_XCR0_BNDCSR;
>> -        xstate_size = max(xstate_size,
>> -                          xstate_offsets[X86_XCR0_BNDCSR_POS] +
>> -                          xstate_sizes[X86_XCR0_BNDCSR_POS]);
>> -    }
>>  
>>      if ( p->feat.avx512f )
>> -    {
>>          xstates |= X86_XCR0_OPMASK | X86_XCR0_ZMM | X86_XCR0_HI_ZMM;
>> -        xstate_size = max(xstate_size,
>> -                          xstate_offsets[X86_XCR0_HI_ZMM_POS] +
>> -                          xstate_sizes[X86_XCR0_HI_ZMM_POS]);
>> -    }
>>  
>>      if ( p->feat.pku )
>> -    {
>>          xstates |= X86_XCR0_PKRU;
>> -        xstate_size = max(xstate_size,
>> -                          xstate_offsets[X86_XCR0_PKRU_POS] +
>> -                          xstate_sizes[X86_XCR0_PKRU_POS]);
>> -    }
>>  
>> -    p->xstate.max_size  =  xstate_size;
>> +    /* Subleaf 0 */
>> +    p->xstate.max_size =
>> +        xstate_uncompressed_size(xstates & ~XSTATE_XSAVES_ONLY);
>>      p->xstate.xcr0_low  =  xstates & ~XSTATE_XSAVES_ONLY;
>>      p->xstate.xcr0_high = (xstates & ~XSTATE_XSAVES_ONLY) >> 32;
>>  
>> +    /* Subleaf 1 */
>>      p->xstate.Da1 = Da1;
>> +    if ( p->xstate.xsavec )
>> +        ecx_mask |= XSTATE_ALIGN64;
>> +
>>      if ( p->xstate.xsaves )
>>      {
>> +        ecx_mask |= XSTATE_XSS;
>>          p->xstate.xss_low   =  xstates & XSTATE_XSAVES_ONLY;
>>          p->xstate.xss_high  = (xstates & XSTATE_XSAVES_ONLY) >> 32;
>>      }
>> -    else
>> -        xstates &= ~XSTATE_XSAVES_ONLY;
>>  
>> -    for ( i = 2; i < min(63UL, ARRAY_SIZE(p->xstate.comp)); ++i )
>> +    /* Subleafs 2+ */
>> +    xstates &= ~XSTATE_FP_SSE;
>> +    BUILD_BUG_ON(ARRAY_SIZE(p->xstate.comp) < 63);
>> +    for_each_set_bit ( i, &xstates, 63 )
>>      {
>> -        uint64_t curr_xstate = 1UL << i;
>> -
>> -        if ( !(xstates & curr_xstate) )
>> -            continue;
>> -
>> -        p->xstate.comp[i].size   = xstate_sizes[i];
>> -        p->xstate.comp[i].offset = xstate_offsets[i];
>> -        p->xstate.comp[i].xss    = curr_xstate & XSTATE_XSAVES_ONLY;
>> -        p->xstate.comp[i].align  = curr_xstate & xstate_align;
> ... for this bit, isn't the move from this ...
>
>> +        /*
>> +         * Pass through size (eax) and offset (ebx) directly.  Visbility of
>> +         * attributes in ecx limited by visible features in Da1.
>> +         */
>> +        p->xstate.raw[i].a = raw_cpu_policy.xstate.raw[i].a;
>> +        p->xstate.raw[i].b = raw_cpu_policy.xstate.raw[i].b;
>> +        p->xstate.raw[i].c = raw_cpu_policy.xstate.raw[i].c & ecx_mask;
> ... to this changing what guests get to see, i.e. (mildly?) incompatible?

No.

The only "rows" in leaf 0xd we expose to guests are AVX, MPX, AVX512 and
PKU  (higher up in this hunk, selecting valid bits in xstates).  None of
these have a non-zero value in ecx.

This is a latent bug until we offer AMX or CET, hence why I wanted to
complete this series before your AMX series goes in.

~Andrew