RE: [PATCH Part2 v6 47/49] *fix for stale per-cpu pointer due to cond_resched during ghcb mapping

Kalra, Ashish
2022-06-24T16:44:25Z

[AMD Official Use Only - General]

Hello Peter,
>>
>> From: Michael Roth <michael.roth@amd.com>
>>
>> Signed-off-by: Michael Roth <michael.roth@amd.com>

>Can you add a commit description here? Is this a fix for existing SEV-ES support or should this be incorporated into a patch in this series which adds this issue?

This actually fixes issues caused due to preemption happening in svm_prepare_switch_to_guest() when kvm_vcpu_map() is called to map in the GHCB before
entering the guest.

This is a temporary fix and what we need to do is to prevent getting preempted after vcpu_enter_guest() has disabled preemption, have some ideas about
using gfn_to_pfn_cache() infrastructure to re-use the already mapped GHCB at guest exit, so that we can avoid calling kvm_vcpu_map() to re-map the
GHCB.

Thanks,
Ashish

> ---
> arch/x86/kvm/svm/svm.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index
> fced6ea423ad..f78e3b1bde0e 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1352,7 +1352,7 @@ static void svm_vcpu_free(struct kvm_vcpu *vcpu)
> static void svm_prepare_switch_to_guest(struct kvm_vcpu *vcpu) {
> struct vcpu_svm *svm = to_svm(vcpu);
> - struct svm_cpu_data *sd = per_cpu(svm_data, vcpu->cpu);
> + struct svm_cpu_data *sd;
>
> if (sev_es_guest(vcpu->kvm))
> sev_es_unmap_ghcb(svm); @@ -1360,6 +1360,10 @@ static
> void svm_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
> if (svm->guest_state_loaded)
> return;
>
> + /* sev_es_unmap_ghcb() can resched, so grab per-cpu pointer afterward. */
> + barrier();
> + sd = per_cpu(svm_data, vcpu->cpu);
> +
> /*
> * Save additional host state that will be restored on VMEXIT (sev-es)
> * or subsequent vmload of host save area.
> --
> 2.25.1
>