Re: [PATCH 2/5] x86/cpu-policy: Add SVM features already used by Xen

[George Dunlap <george.dunlap@xxxxxxxxx>]

On Wed, May 1, 2024 at 11:39 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 01/05/2024 11:00 am, George Dunlap wrote:
> > On Mon, Apr 29, 2024 at 4:16 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
> > wrote:
> >> These will replace svm_feature_flags and the SVM_FEATURE_* constants over 
> >> the
> >> next few changes.  Take the opportunity to rationalise some names.
> >>
> >> Drop the opencoded "inherit from host" logic in calculate_hvm_max_policy() 
> >> and
> >> use 'h'/'!' annotations.  The logic needs to operate on fs, not the policy
> >> object, given its position within the function.
> >>
> >> Drop some trailing whitespace introduced when this block of code was last
> >> moved.
> >>
> >> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> >> ---
> >> CC: Jan Beulich <JBeulich@xxxxxxxx>
> >> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> >> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> >> CC: Xenia Ragiadakou <xenia.ragiadakou@xxxxxxx>
> >> CC: Sergiy Kibrik <Sergiy_Kibrik@xxxxxxxx>
> >> CC: George Dunlap <george.dunlap@xxxxxxxxxx>
> >> CC: Andrei Semenov <andrei.semenov@xxxxxxxx>
> >> CC: Vaishali Thakkar <vaishali.thakkar@xxxxxxxxxx>
> >> ---
> >>  tools/misc/xen-cpuid.c                      | 11 +++++++++++
> >>  xen/arch/x86/cpu-policy.c                   | 17 +++++------------
> >>  xen/include/public/arch-x86/cpufeatureset.h | 14 ++++++++++++++
> >>  xen/tools/gen-cpuid.py                      |  3 +++
> >>  4 files changed, 33 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/tools/misc/xen-cpuid.c b/tools/misc/xen-cpuid.c
> >> index ab09410a05d6..0d01b0e797f1 100644
> >> --- a/tools/misc/xen-cpuid.c
> >> +++ b/tools/misc/xen-cpuid.c
> >> @@ -266,6 +266,17 @@ static const char *const str_m10Ah[32] =
> >>
> >>  static const char *const str_eAd[32] =
> >>  {
> >> +    [ 0] = "npt",                 [ 1] = "v-lbr",
> >> +    [ 2] = "svm-lock",            [ 3] = "nrips",
> >> +    [ 4] = "v-tsc-rate",          [ 5] = "vmcb-cleanbits",
> >> +    [ 6] = "flush-by-asid",       [ 7] = "decode-assist",
> >> +
> >> +    [10] = "pause-filter",
> >> +    [12] = "pause-thresh",
> >> +    /* 14 */                      [15] = "v-loadsave",
> >> +    [16] = "v-gif",
> >> +    /* 18 */                      [19] = "npt-sss",
> >> +    [20] = "v-spec-ctrl",
> >>  };
> >>
> >>  static const char *const str_e1Fa[32] =
> >> diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c
> >> index 4b6d96276399..da4401047e89 100644
> >> --- a/xen/arch/x86/cpu-policy.c
> >> +++ b/xen/arch/x86/cpu-policy.c
> >> @@ -9,7 +9,6 @@
> >>  #include <asm/amd.h>
> >>  #include <asm/cpu-policy.h>
> >>  #include <asm/hvm/nestedhvm.h>
> >> -#include <asm/hvm/svm/svm.h>
> >>  #include <asm/intel-family.h>
> >>  #include <asm/msr-index.h>
> >>  #include <asm/paging.h>
> >> @@ -748,22 +747,16 @@ static void __init calculate_hvm_max_policy(void)
> >>      if ( !cpu_has_vmx )
> >>          __clear_bit(X86_FEATURE_PKS, fs);
> >>
> >> -    /*
> >> +    /*
> >>       * Make adjustments to possible (nested) virtualization features 
> >> exposed
> >>       * to the guest
> >>       */
> >> -    if ( p->extd.svm )
> >> +    if ( test_bit(X86_FEATURE_SVM, fs) )
> >>      {
> >> -        /* Clamp to implemented features which require hardware support. 
> >> */
> >> -        p->extd.raw[0xa].d &= ((1u << SVM_FEATURE_NPT) |
> >> -                               (1u << SVM_FEATURE_LBRV) |
> >> -                               (1u << SVM_FEATURE_NRIPS) |
> >> -                               (1u << SVM_FEATURE_PAUSEFILTER) |
> >> -                               (1u << SVM_FEATURE_DECODEASSISTS));
> >> -        /* Enable features which are always emulated. */
> >> -        p->extd.raw[0xa].d |= (1u << SVM_FEATURE_VMCBCLEAN);
> >> +        /* Xen always emulates cleanbits. */
> >> +        __set_bit(X86_FEATURE_VMCB_CLEANBITS, fs);
> >>      }
> > What about this line, at the end of recalculate_cpuid_policy()?
> >
> >   if ( !p->extd.svm )
> >         p->extd.raw[0xa] = EMPTY_LEAF;
> >
> > Is there a reason to continue to operate directly on the policy here,
> > or would it be better to do this up in the featureset section?
>
> That's still needed.
>
> Otherwise in a !SVM VM you still see svm_rev and nr_asids in a leaf that
> should be all zeroes.

...Uh, yes of course we still need to clear the non-existent CPUID
bits.  I was asking if we should change *how* we should clear them.

In the hunk I responded to, when enabling VMCBCLEAN, we switch from
setting bits in the policy, to setting bits in the featureset.  I was
asking whether it would make sense to do something like

    if !test_bit(X86_FEATURE_SVM, fs)
        fs[FEATURESET_eAd]  = 0;

before the x86_cpu_featureset_to_policy() instead.

 -George