Re: [PATCH v3 8/8] x86: vmx: virtualize split lock detection

Sean Christopherson
2020-03-05T16:49:30Z

On Thu, Mar 05, 2020 at 10:16:40PM +0800, Xiaoyao Li wrote:
> On 3/4/2020 3:30 AM, Sean Christopherson wrote:
> >On Thu, Feb 06, 2020 at 03:04:12PM +0800, Xiaoyao Li wrote:
> >>--- a/arch/x86/kvm/vmx/vmx.c
> >>+++ b/arch/x86/kvm/vmx/vmx.c
> >>@@ -1781,6 +1781,25 @@ static int vmx_get_msr_feature(struct kvm_msr_entry *msr)
> >> }
> >> }
> >>+/*
> >>+ * Note: for guest, feature split lock detection can only be enumerated through
> >>+ * MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT bit. The FMS enumeration is invalid.
> >>+ */
> >>+static inline bool guest_has_feature_split_lock_detect(struct kvm_vcpu *vcpu)
> >>+{
> >>+ return vcpu->arch.core_capabilities & MSR_IA32_CORE_CAPS_SPLIT_LOCK_DETECT;
> >>+}
> >>+
> >>+static inline u64 vmx_msr_test_ctrl_valid_bits(struct kvm_vcpu *vcpu)
> >>+{
> >>+ u64 valid_bits = 0;
> >>+
> >>+ if (guest_has_feature_split_lock_detect(vcpu))
> >>+ valid_bits |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> >>+
> >>+ return valid_bits;
> >>+}
> >>+
> >> /*
> >> * Reads an msr value (of 'msr_index') into 'pdata'.
> >> * Returns 0 on success, non-0 otherwise.
> >>@@ -1793,6 +1812,12 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >> u32 index;
> >> switch (msr_info->index) {
> >>+ case MSR_TEST_CTRL:
> >>+ if (!msr_info->host_initiated &&
> >>+ !guest_has_feature_split_lock_detect(vcpu))
> >>+ return 1;
> >>+ msr_info->data = vmx->msr_test_ctrl;
> >>+ break;
> >> #ifdef CONFIG_X86_64
> >> case MSR_FS_BASE:
> >> msr_info->data = vmcs_readl(GUEST_FS_BASE);
> >>@@ -1934,6 +1959,13 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >> u32 index;
> >> switch (msr_index) {
> >>+ case MSR_TEST_CTRL:
> >>+ if (!msr_info->host_initiated &&
> >
> >Host initiated writes need to be validated against
> >kvm_get_core_capabilities(), otherwise userspace can enable SLD when it's
> >supported in hardware and the kernel, but can't be safely exposed to the
> >guest due to SMT being on.
>
> How about making the whole check like this:
>
> if (!msr_info->host_initiated &&
> (!guest_has_feature_split_lock_detect(vcpu))
> return 1;
>
> if (data & ~vmx_msr_test_ctrl_valid_bits(vcpu))

Whoops, the check on kvm_get_core_capabilities() should be done in
"case MSR_IA32_CORE_CAPS:", i.e. KVM shouldn't let host userspace advertise
split-lock support unless it's allowed by KVM.

Then this code doesn't need to do a check on host_initiated=true.

Back to the original code, I don't think we need to make the existence of
MSR_TEST_CTRL dependent on guest_has_feature_split_lock_detect(), i.e. this
check can simply be:

if (!msr_info->host_initiated &&
(data & ~vmx_msr_test_ctrl_valid_bits(vcpu)))
return 1;

and vmx_get_msr() doesn't need to check anything, i.e. RDMSR always
succeeds. This is actually aligned with real silicon behavior because
MSR_TEST_CTRL exists on older processors, it's just wasn't documented until
we decided to throw in SPLIT_LOCK_AC, e.g. the LOCK# suppression bit is
marked for deprecation in the SDM, which wouldn't be necessary if it didn't
exist :-)

Intel ISA/Feature Year of Removal
TEST_CTRL MSR, bit 31 (MSR address 33H) 2019 onwards

31 Disable LOCK# assertion for split locked access

On my Haswell box:

$ rdmsr 0x33
0
$ wrmsr 0x33 0x20000000
wrmsr: CPU 0 cannot set MSR 0x00000033 to 0x0000000020000000
$ wrmsr 0x33 0x80000000
$ rdmsr 0x33
80000000
$ wrmsr 0x33 0x00000000
$ rdmsr 0x33
0

That way the guest_has_feature_split_lock_detect() helper isn't needed
since its only user is vmx_msr_test_ctrl_valid_bits(), i.e. it can be
open coded there.

> >>+ (!guest_has_feature_split_lock_detect(vcpu) ||
> >>+ data & ~vmx_msr_test_ctrl_valid_bits(vcpu)))
> >>+ return 1;
> >>+ vmx->msr_test_ctrl = data;
> m>+ break;