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

[Jan Beulich <jbeulich@xxxxxxxx>]

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"?

> 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?
(For .xss there's no issue because XSTATE_XSAVES_ONLY is still 0.)

Jan