On 02.11.2022 18:38, Roger Pau Monné wrote:
> On Wed, Nov 02, 2022 at 12:49:17PM +0100, Jan Beulich wrote:
>> On 29.10.2022 15:12, Roger Pau Monne wrote:
>>> --- a/xen/arch/x86/hvm/svm/svm.c
>>> +++ b/xen/arch/x86/hvm/svm/svm.c
>>> @@ -973,6 +973,16 @@ static void cf_check svm_ctxt_switch_from(struct vcpu
>>> *v)
>>>
>>> /* Resume use of ISTs now that the host TR is reinstated. */
>>> enable_each_ist(idt_tables[cpu]);
>>> +
>>> + /*
>>> + * Clear previous guest selection of SSBD if set. Note that
>>> SPEC_CTRL.SSBD
>>> + * is already cleared by svm_vmexit_spec_ctrl.
>>> + */
>>> + if ( v->arch.msrs->virt_spec_ctrl.raw & SPEC_CTRL_SSBD )
>>> + {
>>> + ASSERT(v->domain->arch.cpuid->extd.virt_ssbd);
>>> + amd_set_ssbd(false);
>>> + }
>>> }
>>
>> Aren't you potentially turning off SSBD here just to ...
>>
>>> @@ -1000,6 +1010,13 @@ static void cf_check svm_ctxt_switch_to(struct vcpu
>>> *v)
>>>
>>> if ( cpu_has_msr_tsc_aux )
>>> wrmsr_tsc_aux(v->arch.msrs->tsc_aux);
>>> +
>>> + /* Load SSBD if set by the guest. */
>>> + if ( v->arch.msrs->virt_spec_ctrl.raw & SPEC_CTRL_SSBD )
>>> + {
>>> + ASSERT(v->domain->arch.cpuid->extd.virt_ssbd);
>>> + amd_set_ssbd(true);
>>> + }
>>> }
>>
>> ... turn it on here again? IOW wouldn't switching better be isolated to
>> just svm_ctxt_switch_to(), doing nothing if already in the intended mode?
>
> What if we switch from a HVM vCPU into a PV one? AFAICT then
> svm_ctxt_switch_to() won't get called and we would be running the PV
> guest with the previous HVM domain SSBD selection.
Would that be a problem? Or in other words: What is the intended behavior
for PV? PV domains can control SSBD via SPEC_CTRL (only), so all we need
to guarantee is that we respect their choice there.
>>> --- a/xen/arch/x86/msr.c
>>> +++ b/xen/arch/x86/msr.c
>>> @@ -697,7 +697,15 @@ int guest_wrmsr(struct vcpu *v, uint32_t msr, uint64_t
>>> val)
>>> msrs->spec_ctrl.raw &= ~SPEC_CTRL_SSBD;
>>> }
>>> else
>>> + {
>>> msrs->virt_spec_ctrl.raw = val & SPEC_CTRL_SSBD;
>>> + if ( v == curr )
>>> + /*
>>> + * Propagate the value to hardware, as it won't be context
>>> + * switched on vmentry.
>>> + */
>>
>> I have to admit that I find "on vmentry" in the comment misleading: Reading
>> it I first thought you're still alluding to the old model. Plus I also find
>> the combination of "context switched" and "on vmentry" problematic, as we
>> generally mean something else when we say "context switch".
>
> I had a hard time wording this, because of the Xen/guest vs vCPU
> context switches.
>
> What about:
>
> "Propagate the value to hardware, as it won't we set on guest resume
> path."
Sounds better, thanks (with s/we/be/).
>>> + goto set_reg;
>>
>> It's not clear why you want to use hvm_set_reg() in the first place - the
>> comment says "propagate to hardware", which would mean wrmsrl() in the
>> usual case. Here it would mean a direct call to amd_set_ssbd() imo. That
>> would then also be in line with all other "v == curr" conditionals, none
>> of which apply to any "goto set_reg". ..._set_reg(), aiui, is meant only
>> for use in cases where vCPU state needs updating such that proper state
>> would be loaded later (e.g. during VM entry).
>
> I thought it was better to hide those vendor specific calls in the
> already existing vendor hooks (set_reg). I don't mind calling
> amd_set_ssbd() directly here if that's preferred, it seemed kind of a
> layering violation when we have vendor specific hooks in place.
Well, Andrew of course should correct me if I'm wrong, but my understanding
of the get/set-reg interface is as described. On which grounds I don't see
any layering violation here - doing the call right here is merely a more
involved flavor of wrmsrl().
Jan
