Re: [RFC PATCH v2 04/10] x86: Calculate arch LBR CPUID policies

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.01.2025 09:45, Tu Dinh wrote:
> --- a/xen/arch/x86/cpu-policy.c
> +++ b/xen/arch/x86/cpu-policy.c
> @@ -190,6 +190,16 @@ static void sanitise_featureset(uint32_t *fs)
>      }
>  }
>  
> +static void recalculate_arch_lbr(struct cpu_policy *p)
> +{
> +    if ( p->basic.max_leaf < 0x1c ||
> +         !(cpu_policy_xstates(&host_cpu_policy) & X86_XSS_LBR) ||

A reference to the host policy looks questionable here. If you ...

> +         p->basic.lbr_1Ca.supported_depths == 0)
> +        p->feat.arch_lbr = 0;
> +    if ( !p->feat.arch_lbr )
> +        p->basic.raw[0x1c] = EMPTY_LEAF;
> +}
> +
>  static void recalculate_xstate(struct cpu_policy *p)
>  {
>      uint64_t xstates = XSTATE_FP_SSE;
> @@ -219,6 +229,9 @@ static void recalculate_xstate(struct cpu_policy *p)
>      if ( p->feat.amx_tile )
>          xstates |= X86_XCR0_TILE_CFG | X86_XCR0_TILE_DATA;
>  
> +    if ( p->feat.arch_lbr )
> +        xstates |= X86_XSS_LBR;
> +
>      /* Subleaf 0 */
>      p->xstate.max_size =
>          xstate_uncompressed_size(xstates & ~XSTATE_XSAVES_ONLY);
> @@ -271,6 +284,8 @@ static void recalculate_misc(struct cpu_policy *p)
>  
>      p->basic.raw[0xc] = EMPTY_LEAF;
>  
> +    zero_leaves(p->basic.raw, 0xe, 0x1b);
> +
>      p->extd.e1d &= ~CPUID_COMMON_1D_FEATURES;
>  
>      /* Most of Power/RAS hidden from guests. */
> @@ -630,6 +645,7 @@ static void __init calculate_pv_max_policy(void)
>  
>      sanitise_featureset(fs);
>      x86_cpu_featureset_to_policy(fs, p);
> +    recalculate_arch_lbr(p);
>      recalculate_xstate(p);

... flipped the order of these calls, couldn't you use the respective bit
from the correct policy? Then again you need p->feat.arch_lbr to actually
set X86_XSS_LBR. Still the aspect in recalculate_arch_lbr() wants sorting
somehow.

Jan