Messages in this thread | | | Date | Fri, 20 Jan 2023 11:15:48 -0600 | Subject | Re: [PATCH v3 2/2] kvm: sev: If ccp is busy, report throttled to guest | From | Tom Lendacky <> |
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On 1/19/23 15:34, Dionna Glaze wrote:
Since you're building on SNP hypervisor patches, please keep @Ashish on direct copy.
> The ccp driver can be overloaded even with 1 HZ throttling. The return > value of -EBUSY means that there is no firmware error to report back to > user space, so the guest VM would see this as exitinfo2 = 0. The false > success can trick the guest to update its the message sequence number > when it shouldn't have. > > Instead, when ccp returns -EBUSY, that is reported to userspace as the > throttling return value.
Except the CCP driver doesn't return -EBUSY because it is overloaded. It will simply try to acquire the mutex and continue once it has it.
There are a couple of places that return -EBUSY in the driver for other reasons, as well as other -E* values. It looks like these need to be handled properly by the SNP hypervisor patches so that a "success" isn't reported back.
So this patch isn't necessary, but any -E* return value without having actually called the firmware needs to be handled properly. @Ashish, please work with Dionna on this.
Thanks, Tom
> > Cc: Thomas Lendacky <Thomas.Lendacky@amd.com> > Cc: Paolo Bonzini <pbonzini@redhat.com> > Cc: Joerg Roedel <jroedel@suse.de> > Cc: Peter Gonda <pgonda@google.com> > Cc: Borislav Petkov <bp@alien8.de> > > Signed-off-by: Dionna Glaze <dionnaglaze@google.com> > --- > arch/x86/kvm/svm/sev.c | 18 ++++++++++++++++-- > 1 file changed, 16 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c > index cd9372ce6fc2..7da1cc300d7b 100644 > --- a/arch/x86/kvm/svm/sev.c > +++ b/arch/x86/kvm/svm/sev.c > @@ -3642,7 +3642,14 @@ static void snp_handle_guest_request(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t > goto unlock; > > rc = sev_issue_cmd(kvm, SEV_CMD_SNP_GUEST_REQUEST, &data, &err); > - if (rc) > + > + /* > + * The ccp driver can return -EBUSY if the PSP is overloaded, so signal > + * the request has been throttled. > + */ > + if (rc == -EBUSY) > + rc = SNP_GUEST_REQ_THROTTLED; > + else if (rc) > /* use the firmware error code */ > rc = err; > > @@ -3713,7 +3720,14 @@ static void snp_handle_ext_guest_request(struct vcpu_svm *svm, gpa_t req_gpa, gp > if (sev->snp_certs_len) > data_npages = sev->snp_certs_len >> PAGE_SHIFT; > > - if (rc) { > + /* > + * The ccp driver can return -EBUSY if the PSP is overloaded, so signal > + * the request has been throttled. > + */ > + if (rc == -EBUSY) { > + rc = SNP_GUEST_REQ_THROTTLED; > + goto cleanup; > + } else if (rc) { > /* > * If buffer length is small then return the expected > * length in rbx.
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