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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew