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KVM: SVM: Skip OSVW variable updates if current CPU's errata are a subset

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Elide the OSVW variable updates if the current CPU's set of errata are a
subset of the errata tracked in the global values, i.e. if no update is
needed.  There's no danger of under-reporting errata due to bailing early
as KVM is purely reducing the set of "known fixed" errata.  I.e. a racing
update on a different CPU with _more_ errata doesn't change anything if
the current CPU has the same or fewer errata relative to the status quo.

If another CPU is writing osvw_len, then "len" is guaranteed to be larger
than the new osvw_len and so the osvw_len update would be skipped anyways.

If another CPU is setting new bits in osvw_status, then "status" is
guaranteed to be a subset of the new osvw_status and the bitwise-OR would
be an effective nop anyways.

Link: https://patch.msgid.link/20251113231420.1695919-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2025-11-13 15:14:19 -08:00
parent c65106af83
commit 3b7a320e49
1 changed files with 10 additions and 12 deletions
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22
arch/x86/kvm/svm/svm.c
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@ -424,7 +424,6 @@ static void svm_init_osvw(struct kvm_vcpu *vcpu)
static void svm_init_os_visible_workarounds(void)
{
u64 len, status;
int err;
/*
* Get OS-Visible Workarounds (OSVW) bits.
@ -453,20 +452,19 @@ static void svm_init_os_visible_workarounds(void)
return;
}
err = native_read_msr_safe(MSR_AMD64_OSVW_ID_LENGTH, &len);
if (!err)
err = native_read_msr_safe(MSR_AMD64_OSVW_STATUS, &status);
if (native_read_msr_safe(MSR_AMD64_OSVW_ID_LENGTH, &len) ||
native_read_msr_safe(MSR_AMD64_OSVW_STATUS, &status))
len = status = 0;
if (status == READ_ONCE(osvw_status) && len >= READ_ONCE(osvw_len))
return;
guard(spinlock)(&osvw_lock);
if (err) {
osvw_status = osvw_len = 0;
} else {
if (len < osvw_len)
osvw_len = len;
osvw_status |= status;
osvw_status &= (1ULL << osvw_len) - 1;
}
if (len < osvw_len)
osvw_len = len;
osvw_status |= status;
osvw_status &= (1ULL << osvw_len) - 1;
}
static bool __kvm_is_svm_supported(void)