Re: [PATCH 4/5] x86/vmx: handle no model-specific LBR presence

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.05.2022 15:37, Roger Pau Monne wrote:
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -3007,6 +3007,8 @@ static const struct lbr_info {
>      { MSR_GM_LASTBRANCH_0_FROM_IP,  NUM_MSR_GM_LASTBRANCH_FROM_TO },
>      { MSR_GM_LASTBRANCH_0_TO_IP,    NUM_MSR_GM_LASTBRANCH_FROM_TO },
>      { 0, 0 }
> +}, no_lbr[] = {
> +    {0, 0}
>  };

Instead of introducing this and ...

> @@ -3070,6 +3072,21 @@ static const struct lbr_info *last_branch_msr_get(void)
>          /* Goldmont */
>          case 0x5c: case 0x5f:
>              return gm_lbr;
> +
> +        default:
> +            if ( cpu_has_pdcm )
> +            {
> +                uint64_t cap;
> +
> +                rdmsrl(MSR_IA32_PERF_CAPABILITIES, cap);
> +                if ( (cap & MSR_IA32_PERF_CAP_LBR_FORMAT) == 0x3f )
> +                    /*
> +                     * On processors that do not support model-specific LBRs,
> +                     * PERF_CAPABILITIES.LBR_FMT will have the value 0x3f.
> +                     */
> +                    return no_lbr;

... doing this MSR read every time, can't you store a mask value
once during boot, which you apply to msr_content ...

> @@ -3521,6 +3538,8 @@ static int cf_check vmx_msr_write_intercept(
>                  return X86EMUL_OKAY;
>              }
>  
> +            if ( lbr->count )
> +            {
>              for ( ; lbr->count; lbr++ )
>              {
>                  unsigned int i;

... ahead of the bigger if() enclosing this code (thus also avoiding
the need to re-indent)?

Jan