Re: [PATCH 5/5] KVM: VMX: Always honor guest PAT on CPUs that support self-snoop

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Vitaly Kuznetsov <vkuznets@redhat.com> writes:

> Sean Christopherson <seanjc@google.com> writes:
>
>> On Fri, Aug 30, 2024, Vitaly Kuznetsov wrote:
>>> Gerd Hoffmann <kraxel@redhat.com> writes:
>>> 
>>> >> Necroposting!
>>> >> 
>>> >> Turns out that this change broke "bochs-display" driver in QEMU even
>>> >> when the guest is modern (don't ask me 'who the hell uses bochs for
>>> >> modern guests', it was basically a configuration error :-). E.g:
>>> >
>>> > qemu stdvga (the default display device) is affected too.
>>> >
>>> 
>>> So far, I was only able to verify that the issue has nothing to do with
>>> OVMF and multi-vcpu, it reproduces very well with
>>> 
>>> $ qemu-kvm -machine q35,accel=kvm,kernel-irqchip=split -name guest=c10s
>>> -cpu host -smp 1 -m 16384 -drive file=/var/lib/libvirt/images/c10s-bios.qcow2,if=none,id=drive-ide0-0-0
>>> -device ide-hd,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0,bootindex=1
>>> -vnc :0 -device VGA -monitor stdio --no-reboot
>>> 
>>> Comparing traces of working and broken cases, I couldn't find anything
>>> suspicious but I may had missed something of course. For now, it seems
>>> like a userspace misbehavior resulting in a segfault.
>>
>> Guest userspace?
>>
>
> Yes? :-) As Gerd described, video memory is "mapped into userspace so
> the wayland / X11 display server can software-render into the buffer"
> and it seems that wayland gets something unexpected in this memory and
> crashes. 

Also, I don't know if it helps or not, but out of two hunks in
377b2f359d1f, it is the vmx_get_mt_mask() one which brings the
issue. I.e. the following is enough to fix things:

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index f18c2d8c7476..733a0c45d1a6 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7659,13 +7659,11 @@ u8 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
 
        /*
         * Force WB and ignore guest PAT if the VM does NOT have a non-coherent
-        * device attached and the CPU doesn't support self-snoop.  Letting the
-        * guest control memory types on Intel CPUs without self-snoop may
-        * result in unexpected behavior, and so KVM's (historical) ABI is to
-        * trust the guest to behave only as a last resort.
+        * device attached.  Letting the guest control memory types on Intel
+        * CPUs may result in unexpected behavior, and so KVM's ABI is to trust
+        * the guest to behave only as a last resort.
         */
-       if (!static_cpu_has(X86_FEATURE_SELFSNOOP) &&
-           !kvm_arch_has_noncoherent_dma(vcpu->kvm))
+       if (!kvm_arch_has_noncoherent_dma(vcpu->kvm))
                return (MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT) | VMX_EPT_IPAT_BIT;
 
        return (MTRR_TYPE_WRBACK << VMX_EPT_MT_EPTE_SHIFT);


-- 
Vitaly