Commit 02626b6a authored by Marcelo Tosatti's avatar Marcelo Tosatti Committed by Avi Kivity
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KVM: x86: fix kvm_write_tsc() TSC matching thinko 



kvm_write_tsc() converts from guest TSC to microseconds, not nanoseconds
as intended. The result is that the window for matching is 1000 seconds,
not 1 second.

Microsecond precision is enough for checking whether the TSC write delta
is within the heuristic values, so use it instead of nanoseconds.

Noted by Avi Kivity.

Signed-off-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: default avatarAvi Kivity <avi@redhat.com>
parent b74f05d6

arch/x86/kvm/x86.c

+10 −9
Original line number Diff line number Diff line
@@ -1025,7 +1025,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data) 
	struct kvm *kvm = vcpu->kvm;

	u64 offset, ns, elapsed;

	unsigned long flags;

	s64 nsdiff;

	s64 usdiff;



	raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);

	offset = kvm_x86_ops->compute_tsc_offset(vcpu, data);
@@ -1033,18 +1033,19 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data) 
	elapsed = ns - kvm->arch.last_tsc_nsec;



	/* n.b - signed multiplication and division required */

	nsdiff = data - kvm->arch.last_tsc_write;

	usdiff = data - kvm->arch.last_tsc_write;

#ifdef CONFIG_X86_64

	nsdiff = (nsdiff * 1000) / vcpu->arch.virtual_tsc_khz;

	usdiff = (usdiff * 1000) / vcpu->arch.virtual_tsc_khz;

#else

	/* do_div() only does unsigned */

	asm("idivl %2; xor %%edx, %%edx"

	    : "=A"(nsdiff)

	    : "A"(nsdiff * 1000), "rm"(vcpu->arch.virtual_tsc_khz));

	    : "=A"(usdiff)

	    : "A"(usdiff * 1000), "rm"(vcpu->arch.virtual_tsc_khz));

#endif

	nsdiff -= elapsed;

	if (nsdiff < 0)

		nsdiff = -nsdiff;

	do_div(elapsed, 1000);

	usdiff -= elapsed;

	if (usdiff < 0)

		usdiff = -usdiff;



	/*

	 * Special case: TSC write with a small delta (1 second) of virtual
@@ -1056,7 +1057,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data) 
	 * compensation code attempt to catch up if we fall behind, but

	 * it's better to try to match offsets from the beginning.

         */

	if (nsdiff < NSEC_PER_SEC &&

	if (usdiff < USEC_PER_SEC &&

	    vcpu->arch.virtual_tsc_khz == kvm->arch.last_tsc_khz) {

		if (!check_tsc_unstable()) {

			offset = kvm->arch.cur_tsc_offset;

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