Re: [PATCH 7/7] KVM: SVM: Share more code between pre_sev_run() and pre_svm_run()

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Thu, Mar 13, 2025 at 09:55:40PM +0000, Yosry Ahmed wrote:
> pre_svm_run() and pre_sev_run() now do some redundant work, and the
> control flow is not super clear. Specifically:
> - Both functions check if the ASID in the VMCB is the expected one.
> - Both functions check if the vCPU moved to a different physical CPU.
> - Both functions issue an ASID TLB flush if needed.
> 
> Pass the ASID and whether or not SEV requires a TLB flush from
> pre_sev_run() to pre_svm_run(), and use the logic there instead.
> pre_sev_run() now only performs SEV-specific checks.
> 
> Note that pre_sev_run() used svm->vcpu.arch.last_vmentry_cpu to check if
> the vCPU moved to a different physical CPU, while pre_svm_run uses
> svm->current_vmcb->cpu. The former tracks the CPU per vCPU, while the
> latter tracks it per VMCB. For SEV, they both should be equivalent since
> there is a single VMCB per-vCPU (nested is not supported).
> 
> Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
> ---
>  arch/x86/kvm/svm/sev.c | 27 ++++++++++-----------------
>  arch/x86/kvm/svm/svm.c | 10 ++++++----
>  arch/x86/kvm/svm/svm.h |  2 +-
>  3 files changed, 17 insertions(+), 22 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 1ee04d6b9356b..607139757f8ff 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3451,11 +3451,11 @@ void sev_es_unmap_ghcb(struct vcpu_svm *svm)
>  	svm->sev_es.ghcb = NULL;
>  }
>  
> -int pre_sev_run(struct vcpu_svm *svm, int cpu)
> +int pre_sev_run(struct vcpu_svm *svm, unsigned int *asid, bool *need_flush)
>  {
> +	int cpu = svm->vcpu.cpu;
>  	struct svm_cpu_data *sd = per_cpu_ptr(&svm_data, cpu);
>  	struct kvm *kvm = svm->vcpu.kvm;
> -	unsigned int asid = sev_get_asid(kvm);
>  
>  	/*
>  	 * Reject KVM_RUN if userspace attempts to run the vCPU with an invalid
> @@ -3465,24 +3465,17 @@ int pre_sev_run(struct vcpu_svm *svm, int cpu)
>  	if (sev_es_guest(kvm) && !VALID_PAGE(svm->vmcb->control.vmsa_pa))
>  		return -EINVAL;
>  
> -	if (WARN_ON_ONCE(svm->vmcb->control.asid != asid)) {
> -		svm->vmcb->control.asid = asid;
> -		vmcb_mark_dirty(svm->vmcb, VMCB_ASID);
> -	}
> -
>  	/*
> -	 * Flush guest TLB:
> -	 *
> -	 * 1) when different VMCB for the same ASID is to be run on the same host CPU.
> -	 * 2) or this VMCB was executed on different host CPU in previous VMRUNs.
> +	 * Flush the guest TLB when a difference VMCB for the same ASID is to be
> +	 * run on the same host CPU. The caller will also flush the TLB if the
> +	 * VMCB was executed on a different host CPU in previous VMRUNs.
>  	 */
> -	if (sd->sev_vmcbs[asid] == svm->vmcb &&
> -	    svm->vcpu.arch.last_vmentry_cpu == cpu)
> -		return 0;
> +	*asid = sev_get_asid(kvm);
> +	if (sd->sev_vmcbs[*asid] != svm->vmcb) {
> +		sd->sev_vmcbs[*asid] = svm->vmcb;
> +		*need_flush = true;
> +	}
>  
> -	sd->sev_vmcbs[asid] = svm->vmcb;
> -	svm_vmcb_set_flush_asid(svm->vmcb);
> -	vmcb_mark_dirty(svm->vmcb, VMCB_ASID);
>  	return 0;
>  }
>  
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index c5e2733fb856d..6b338d84e7b93 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3615,21 +3615,23 @@ static int pre_svm_run(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
>  	struct vcpu_svm *svm = to_svm(vcpu);
> +	unsigned int asid = kvm_svm->asid;
> +	bool sev_need_flush = false;
> +
> +	if (sev_guest(vcpu->kvm) && pre_sev_run(svm, &asid, &sev_need_flush))
> +		return -1;
>  
>  	/*
>  	 * If the previous VMRUN of the VMCB occurred on a different physical
>  	 * CPU, then mark the VMCB dirty and flush the ASID.  Hardware's
>  	 * VMCB clean bits are per logical CPU, as are KVM's ASID assignments.
>  	 */
> -	if (unlikely(svm->current_vmcb->cpu != vcpu->cpu)) {
> +	if (unlikely(sev_need_flush || svm->current_vmcb->cpu != vcpu->cpu)) {
>  		svm_vmcb_set_flush_asid(svm->vmcb);
>  		vmcb_mark_all_dirty(svm->vmcb);
>  		svm->current_vmcb->cpu = vcpu->cpu;
>          }
>  
> -	if (sev_guest(vcpu->kvm))
> -		return pre_sev_run(svm, vcpu->cpu);
> -
>  	/* Flush the ASID on every VMRUN if kvm_svm->asid allocation failed */
>  	if (unlikely(!kvm_svm->asid))

This should now check 'asid' instead of 'kvm_svm->asid'.

Same for the WARN below.

>  		svm_vmcb_set_flush_asid(svm->vmcb);
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 4c6664ba4048d..f25e99c79d07d 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -754,7 +754,7 @@ void avic_refresh_virtual_apic_mode(struct kvm_vcpu *vcpu);
>  
>  /* sev.c */
>  
> -int pre_sev_run(struct vcpu_svm *svm, int cpu);
> +int pre_sev_run(struct vcpu_svm *svm, unsigned int *asid, bool *need_flush);
>  void sev_init_vmcb(struct vcpu_svm *svm);
>  void sev_vcpu_after_set_cpuid(struct vcpu_svm *svm);
>  int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in);
> -- 
> 2.49.0.rc1.451.g8f38331e32-goog
>