Re: [PATCH 1/3] KVM: x86: fix reporting of AMD speculation bug CPUID leaf

[Eric Biggers]

Hi Paolo,

On Wed, Aug 21, 2019 at 10:26:40AM +0200, Paolo Bonzini wrote:
> The AMD_* bits have to be set from the vendor-independent
> feature and bug flags, because KVM_GET_SUPPORTED_CPUID does not care
> about the vendor and they should be set on Intel processors as well.
> On top of this, SSBD, STIBP and AMD_SSB_NO bit were not set, and
> VIRT_SSBD does not have to be added manually because it is a
> cpufeature that comes directly from the host's CPUID bit.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  arch/x86/kvm/cpuid.c | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 22c2720cd948..43caeb6059b9 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -729,18 +729,23 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
>  			g_phys_as = phys_as;
>  		entry->eax = g_phys_as | (virt_as << 8);
>  		entry->edx = 0;
> +		entry->ebx &= kvm_cpuid_8000_0008_ebx_x86_features;
> +		cpuid_mask(&entry->ebx, CPUID_8000_0008_EBX);
>  		/*
> -		 * IBRS, IBPB and VIRT_SSBD aren't necessarily present in
> -		 * hardware cpuid
> +		 * AMD has separate bits for each SPEC_CTRL bit.
> +		 * arch/x86/kernel/cpu/bugs.c is kind enough to
> +		 * record that in cpufeatures so use them.
>  		 */
> -		if (boot_cpu_has(X86_FEATURE_AMD_IBPB))
> +		if (boot_cpu_has(X86_FEATURE_IBPB))
>  			entry->ebx |= F(AMD_IBPB);
> -		if (boot_cpu_has(X86_FEATURE_AMD_IBRS))
> +		if (boot_cpu_has(X86_FEATURE_IBRS))
>  			entry->ebx |= F(AMD_IBRS);
> -		if (boot_cpu_has(X86_FEATURE_VIRT_SSBD))
> -			entry->ebx |= F(VIRT_SSBD);
> -		entry->ebx &= kvm_cpuid_8000_0008_ebx_x86_features;
> -		cpuid_mask(&entry->ebx, CPUID_8000_0008_EBX);
> +		if (boot_cpu_has(X86_FEATURE_STIBP))
> +			entry->ebx |= F(AMD_STIBP);
> +		if (boot_cpu_has(X86_FEATURE_SSBD))
> +			entry->ebx |= F(AMD_SSBD);
> +		if (!boot_cpu_has_bug(X86_BUG_SPEC_STORE_BYPASS))
> +			entry->ebx |= F(AMD_SSB_NO);
>  		/*
>  		 * The preference is to use SPEC CTRL MSR instead of the
>  		 * VIRT_SPEC MSR.

This patch started causing a warning about an unchecked MSR access, when
starting a VM.

Processor is: "AMD Ryzen Threadripper 1950X 16-Core Processor"

The warning appears both in the host and guest kernel logs.

On the host:

	[   12.121802] unchecked MSR access error: RDMSR from 0x48 at rIP: 0xffffffff8b049765 (svm_vcpu_run+0x6a5/0x720)
	[   12.121806] Call Trace:
	[   12.121812]  ? kvm_arch_vcpu_ioctl_run+0x902/0x1b70
	[   12.121814]  ? kvm_vm_ioctl_irq_line+0x1e/0x30
	[   12.121817]  ? kvm_vcpu_ioctl+0x21e/0x560
	[   12.121821]  ? vfs_writev+0xc0/0xf0
	[   12.121824]  ? do_vfs_ioctl+0x41d/0x690
	[   12.121826]  ? ksys_ioctl+0x59/0x90
	[   12.121827]  ? __x64_sys_ioctl+0x11/0x20
	[   12.121828]  ? do_syscall_64+0x43/0x130
	[   12.121832]  ? entry_SYSCALL_64_after_hwframe+0x44/0xa9

On the guest:

	[    0.799090] unchecked MSR access error: WRMSR to 0x48 (tried to write 0x0000000000000004) at rIP: 0xffffffff81028272 (speculation_ctrl_update+0x132/0x2c0)
	[    0.801823] Call Trace:
	[    0.801831]  ? seccomp_set_mode_filter+0x18d/0x800
	[    0.801833]  speculation_ctrl_update_current+0x21/0x30
	[    0.801837]  task_update_spec_tif+0x1d/0x20
	[    0.801839]  ssb_prctl_set+0xb5/0xd0
	[    0.801841]  arch_seccomp_spec_mitigate+0x2a/0x50
	[    0.801843]  seccomp_set_mode_filter+0x788/0x800
	[    0.801845]  do_seccomp+0x34/0x200
	[    0.801849]  __x64_sys_seccomp+0x15/0x20
	[    0.801852]  do_syscall_64+0x4a/0x1f0
	[    0.809349]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
	[    0.810548] RIP: 0033:0x7f431db92e9d
	[    0.811528] Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d b3 5f 0c 00 f7 d8 64 89 01 48
	[    0.814754] RSP: 002b:00007ffca5506788 EFLAGS: 00000246 ORIG_RAX: 000000000000013d
	[    0.816075] RAX: ffffffffffffffda RBX: 0000556956c07580 RCX: 00007f431db92e9d
	[    0.817367] RDX: 0000556956c023e0 RSI: 0000000000000000 RDI: 0000000000000001
	[    0.818698] RBP: 0000556956c023e0 R08: 0000000000000007 R09: 0000556956b73730
	[    0.819948] R10: 0000556956b7101a R11: 0000000000000246 R12: 000000000000002d
	[    0.821184] R13: 0040000000002001 R14: 00007f431d9b4898 R15: 0000000000000000

The VM still boots though.

I've actually been seeing this for a while but haven't had a chance to bisect it
until now.

Reverting the commit (4c6903a0f9d76) on mainline makes the warnings go away.

Any ideas?  Presumably something isn't working as intended.

- Eric