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@xxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Joerg Roedel <jroedel@xxxxxxx>
Cc: Peter Gonda <pgonda@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Dionna Glaze <dionnaglaze@xxxxxxxxxx>
---
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.
