Re: [Xen-devel] [PATCH] x86/svm: Drop support for AMD's Lightweight Profiling

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.05.19 at 12:13, <andrew.cooper3@xxxxxxxxxx> wrote:
> Lightweight Profiling was introduced in Bulldozer (Fam15h), but was dropped
> from Zen (Fam17h) processors.  Furthermore, LWP was dropped from Fam15/16 CPUs
> when IBPB for Spectre v2 was introduced in microcode, owing to LWP not being
> used in practice.
> 
> As a result, CPUs which are operating within specification (i.e. with up to
> date microcode) no longer have this feature, and therefore are not using it.
> 
> Drop support from Xen.  The main motivation here is to remove unnecessary
> complexity from CPUID handling, but it also tidies up the SVM code nicely.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

> --- a/xen/include/public/arch-x86/cpufeatureset.h
> +++ b/xen/include/public/arch-x86/cpufeatureset.h
> @@ -176,7 +176,7 @@ XEN_CPUFEATURE(IBS,           3*32+10) /*   Instruction 
> Based Sampling */
>  XEN_CPUFEATURE(XOP,           3*32+11) /*A  extended AVX instructions */
>  XEN_CPUFEATURE(SKINIT,        3*32+12) /*   SKINIT/STGI instructions */
>  XEN_CPUFEATURE(WDT,           3*32+13) /*   Watchdog timer */
> -XEN_CPUFEATURE(LWP,           3*32+15) /*S  Light Weight Profiling */
> +XEN_CPUFEATURE(LWP,           3*32+15) /*   Light Weight Profiling */

Strictly speaking this is not permitted (see the other thread on
this being part of the public interface). But of course strictly
speaking it was also not permitted for AMD to remove the
feature in a ucode update (I wonder btw whether the insns
indeed #UD now on Fam15/16).

Jan



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