On Thu, Jan 28, 2021, Paolo Bonzini wrote:

Userspace that does not know about KVM_GET_MSR_FEATURE_INDEX_LIST will generally use the default value for MSR_IA32_ARCH_CAPABILITIES. When this happens and the host has tsx=on, it is possible to end up with virtual machines that have HLE and RTM disabled, but TSX_CTRL disabled.

Thos wording is confusing the heck out of me. I think what you're saying is "but TSX disabled in the guest via TSX_CTRL". I read "but TSX_CTRL disabled" as saying the the TSX_CTRL itself was disabled/unsupported.

If the fleet is then switched to tsx=off, kvm_get_arch_capabilities() will clear the ARCH_CAP_TSX_CTRL_MSR bit and it will not be possible to use the tsx=off as migration destinations, even though the guests indeed do not have TSX enabled.

When tsx=off is used, however, we know that guests will not have HLE and RTM (or if userspace sets bogus CPUID data, we do not expect HLE and RTM to work in guests). Therefore we can keep TSX_CTRL_RTM_DISABLE set for the entire life of the guests and save MSR reads and writes on KVM_RUN and in the user return notifiers.

Cc: stable@vger.kernel.org Fixes: cbbaa2727aa3 ("KVM: x86: fix presentation of TSX feature in ARCH_CAPABILITIES") Signed-off-by: Paolo Bonzini pbonzini@redhat.com

---

arch/x86/kvm/vmx/vmx.c | 12 +++++++++++- arch/x86/kvm/x86.c | 2 +- 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index cc60b1fc3ee7..80491a729408 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -6863,8 +6863,18 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu) * No need to pass TSX_CTRL_CPUID_CLEAR through, so * let's avoid changing CPUID bits under the host * kernel's feet.

• 	 *
  

• 	 * If the host disabled RTM, we may still need TSX_CTRL
  

• 	 * to be supported in the guest; for example the guest
  

• 	 * could have been created on a tsx=on host with hle=0,
  

• 	 * rtm=0, tsx_ctrl=1 and later migrate to a tsx=off host.
  

• 	 * In that case however do not change the value on the host,
  

• 	 * so that TSX remains always disabled.
  

Oof, can you reword this to clarify what "the value" refers to? The previous paragraphs talks about TSX_CTRL_CPUID_CLEAR, and the obvious "value" in the code is also TSX_CTRL_CPUID_CLEAR, and so I thought the comment was saying "don't change the value of CPUID_CLEAR", which is non-sensical because that's the the RTM-enabled case does...

	 */

• 	vmx->guest_uret_msrs[j].mask = ~(u64)TSX_CTRL_CPUID_CLEAR;
  

• 	if (boot_cpu_has(X86_FEATURE_RTM))
  

• 		vmx->guest_uret_msrs[j].mask = ~(u64)TSX_CTRL_CPUID_CLEAR;
  

• 	else
  

• 		vmx->guest_uret_msrs[j].mask = 0;
  

IMO, this is an unnecessarily confusing way to "remove" the user return MSR. Changing the ordering to do a 'continue' would also provide a separate chunk of code for the new comment. And maybe replace the switch with an if-statement to avoid a 'continue' buried in a switch?

vmx->guest_uret_msrs[j].slot = i; vmx->guest_uret_msrs[j].data = 0; if (index == MSR_IA32_TSX_CTRL) { /* Fancy new comment here. */ if (!boot_cpu_has(X86_FEATURE_RTM)) continue;

/* * No need to pass TSX_CTRL_CPUID_CLEAR through, so * let's avoid changing CPUID bits under the host * kernel's feet. */ vmx->guest_uret_msrs[j].mask = ~(u64)TSX_CTRL_CPUID_CLEAR; } else { vmx->guest_uret_msrs[j].mask = -1ull;

}

	break;
default:
	vmx->guest_uret_msrs[j].mask = -1ull;

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 76bce832cade..15733013b266 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -1401,7 +1401,7 @@ static u64 kvm_get_arch_capabilities(void)

This comments needs to be rewritten, it reflects the old behavior of exposing the feature iff RTM/TSC is supported by the host.

*	  This lets the guest use VERW to clear CPU buffers.
*/

if (!boot_cpu_has(X86_FEATURE_RTM))

• data &= ~(ARCH_CAP_TAA_NO | ARCH_CAP_TSX_CTRL_MSR);
  

• data &= ~ARCH_CAP_TAA_NO;
  

  else if (!boot_cpu_has_bug(X86_BUG_TAA)) data |= ARCH_CAP_TAA_NO;

2.26.2