Re: [PATCH v3 1/3] KVM: nVMX: Handle dynamic MSR intercept toggling

Sean Christopherson
2021-11-08T17:25:01Z

On Mon, Nov 08, 2021, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@google.com> writes:
> > @@ -6749,7 +6686,9 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
> > * If the L02 MSR bitmap does not intercept the MSR, then we need to
> > * save it.
> > */
> > - if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
> > + if (unlikely(cpu_has_vmx_msr_bitmap() &&
> > + vmx_test_msr_bitmap_write(vmx->loaded_vmcs->msr_bitmap,
> > + MSR_IA32_SPEC_CTRL)))

Ugh, I inverted the check, '1' == intercept. IIRC, I open coded the intercept
check because SPEC_CTRL is really the only case where should be reading _only_
the current VMCS's MSR bitmap.

I'll spin a new version of the series and test with SPEC_CTRL disabled in a VM,
and maybe revist my reasoning for this.

Thanks!

> > vmx->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
>
> I smoke-tested this patch by running (unrelated) selftests when I tried
> to put in into my 'Enlightened MSR Bitmap v4' series and my dmesg got
> flooded with:
>
> [ 87.210214] unchecked MSR access error: RDMSR from 0x48 at rIP: 0xffffffffc04e0284 (native_read_msr+0x4/0x30 [kvm_intel])
> [ 87.210325] Call Trace:
> [ 87.210355] vmx_vcpu_run+0xcc7/0x12b0 [kvm_intel]
> [ 87.210405] ? vmx_prepare_switch_to_guest+0x138/0x1f0 [kvm_intel]
> [ 87.210466] vcpu_enter_guest+0x98c/0x1380 [kvm]
> [ 87.210631] ? vmx_vcpu_put+0x2e/0x1f0 [kvm_intel]
> [ 87.210678] ? vmx_vcpu_load+0x21/0x60 [kvm_intel]
> [ 87.210729] kvm_arch_vcpu_ioctl_run+0xdf/0x580 [kvm]
> [ 87.210844] kvm_vcpu_ioctl+0x274/0x660 [kvm]
> [ 87.210950] __x64_sys_ioctl+0x83/0xb0
> [ 87.210996] do_syscall_64+0x3b/0x90
> [ 87.211039] entry_SYSCALL_64_after_hwframe+0x44/0xae
> [ 87.211093] RIP: 0033:0x7f6ef7f9a307
> [ 87.211134] Code: 44 00 00 48 8b 05 69 1b 2d 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 39 1b 2d 00 f7 d8 64 89 01 48
> [ 87.211293] RSP: 002b:00007ffcacfb3b18 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> [ 87.211367] RAX: ffffffffffffffda RBX: 0000000000a2f300 RCX: 00007f6ef7f9a307
> [ 87.211434] RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000007
> [ 87.211500] RBP: 0000000000000000 R08: 000000000040e769 R09: 0000000000000000
> [ 87.211559] R10: 0000000000a2f001 R11: 0000000000000246 R12: 0000000000a2d010
> [ 87.211622] R13: 0000000000a2d010 R14: 0000000000402a15 R15: 00000000ffff0ff0
> [ 87.212520] Call Trace:
> [ 87.212597] vmx_vcpu_run+0xcc7/0x12b0 [kvm_intel]
> [ 87.212683] ? vmx_prepare_switch_to_guest+0x138/0x1f0 [kvm_intel]
> [ 87.212789] vcpu_enter_guest+0x98c/0x1380 [kvm]
> [ 87.213059] ? vmx_vcpu_put+0x2e/0x1f0 [kvm_intel]
> [ 87.213141] ? schedule+0x44/0xa0
> [ 87.213200] kvm_arch_vcpu_ioctl_run+0xdf/0x580 [kvm]
> [ 87.213428] kvm_vcpu_ioctl+0x274/0x660 [kvm]
> [ 87.213633] __x64_sys_ioctl+0x83/0xb0
> [ 87.213705] do_syscall_64+0x3b/0x90
> [ 87.213766] entry_SYSCALL_64_after_hwframe+0x44/0xae
> ...
>
> this was an old 'E5-2603 v3' CPU. Any idea what's wrong?