Re: [PATCH] KVM: SVM: Add support for AMD's OSVW feature in guests

From: Boris Ostrovsky
Date: Tue Dec 27 2011 - 01:52:04 EST

On 12/26/11 08:53, Avi Kivity wrote:
On 12/19/2011 07:46 PM, Boris Ostrovsky wrote:
From: Boris Ostrovsky<boris.ostrovsky@xxxxxxx>

In some cases guests should not provide workarounds for errata even when the
physical processor is affected. For example, because of erratum 400 on family
10h processors a Linux guest will read an MSR (resulting in VMEXIT) before
going to idle in order to avoid getting stuck in a non-C0 state. This is not
necessary: HLT and IO instructions are intercepted and therefore there is no
reason for erratum 400 workaround in the guest.

This patch allows us to present a guest with certain errata as fixed,
regardless of the state of actual hardware.

+static int svm_handle_osvw(struct kvm_vcpu *vcpu,
+ uint32_t msr, uint64_t *val)
+ struct kvm_cpuid_entry2 *cpuid_entry;
+ /* Guest OSVW support */
+ cpuid_entry = kvm_find_cpuid_entry(vcpu, 0x80000001, 0);
+ if (!cpuid_entry || !(cpuid_entry->ecx& bit(X86_FEATURE_OSVW)))
+ return -1;
+ /*
+ * Guests should see errata 400 and 415 as fixed (assuming that
+ * HLT and IO instructions are intercepted).
+ */
+ if (msr == MSR_AMD64_OSVW_ID_LENGTH)
+ *val = (osvw_len>= 3) ? (osvw_len) : 3;
+ else {
+ *val = osvw_status& ~(6ULL);
+ if (osvw_len == 0&& boot_cpu_data.x86 == 0x10)
+ /* Mark erratum 298 as present */
+ *val |= 1;
+ }
+ return 0;

Please move this to common code, to support cross-vendor migration.

OK. (Note though that the OSVW registers are typically checked during system boot so if you migrate a running guest it is unlikely that it will read them again)

static int has_svm(void)
const char *msg;
@@ -620,6 +656,37 @@ static int svm_hardware_enable(void *garbage)
__get_cpu_var(current_tsc_ratio) = TSC_RATIO_DEFAULT;

+ /*
+ * Get OSVW bits.
+ *
+ * Note that it is possible to have a system with mixed processor
+ * revisions and therefore different OSVW bits. If bits are not the same
+ * on different processors then choose the worst case (i.e. if erratum
+ * is present on one processor and not on another then assume that the
+ * erratum is present everywhere).
+ */
+ if (cpu_has(&boot_cpu_data, X86_FEATURE_OSVW)) {
+ uint64_t len, status;
+ int err;
+ len = native_read_msr_safe(MSR_AMD64_OSVW_ID_LENGTH,&err);
+ if (!err)
+ status = native_read_msr_safe(MSR_AMD64_OSVW_STATUS,
+ &err);
+ spin_lock(&svm_lock);
+ if (err)
+ osvw_status = osvw_len = 0;
+ else {
+ if (len< osvw_len)
+ osvw_len = len;

This implies that if a bit is inside len, then the OS must apply the

Almost: when osvw bit is inside len then OS workaround is applied if the bit is set. And if the bit is outside len then OS workaround should always be applied.

+ osvw_status |= status;
+ osvw_status&= (1ULL<< osvw_len) - 1;
+ }
+ spin_unlock(&svm_lock);
+ }

@@ -3092,6 +3162,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, unsigned ecx, u64 data)
pr_unimpl(vcpu, "unimplemented wrmsr: 0x%x data 0x%llx\n", ecx, data);
+ /* Writes are ignored */
+ break;
return kvm_set_msr_common(vcpu, ecx, data);

Best to allow writes, the manual says writes are allowed for bios code,
and the OS should just avoid it.

BIOS is usually the one who sets these bits and OS indeed shouldn't try to write the registers.

The reason I decided to ignore the writes is for cases when we are on a buggy BIOS and a guest writing through to the registers can alter system state.

Not sure what to do here about live migration, since the guest will not
adjust its behaviour. Should management software read those MSRs from
userspace and check that they're consistent across a cluster?

Either that or start the guest on the most "broken" processor (assuming that OSVW registers are only read during boot).

Alternatively the management SW would set emulated values of the MSRs for all systems in the cluster but I am not sure whether this is supported yet.


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