Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by VM instructions

[Sean Christopherson <seanjc@google.com>]

On Tue, Jan 12, 2021, Andy Lutomirski wrote:
> 
> > On Jan 12, 2021, at 7:46 AM, Bandan Das <bsd@redhat.com> wrote:
> > 
> > Andy Lutomirski <luto@amacapital.net> writes:
> > ...
> >>>>>> #endif diff --git a/arch/x86/kvm/mmu/mmu.c
> >>>>>> b/arch/x86/kvm/mmu/mmu.c index 6d16481aa29d..c5c4aaf01a1a 100644
> >>>>>> --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@
> >>>>>> -50,6 +50,7 @@ #include <asm/io.h> #include <asm/vmx.h> #include
> >>>>>> <asm/kvm_page_track.h> +#include <asm/e820/api.h> #include
> >>>>>> "trace.h"
> >>>>>> 
> >>>>>> extern bool itlb_multihit_kvm_mitigation; @@ -5675,6 +5676,12 @@
> >>>>>> void kvm_mmu_slot_set_dirty(struct kvm *kvm, }
> >>>>>> EXPORT_SYMBOL_GPL(kvm_mmu_slot_set_dirty);
> >>>>>> 
> >>>>>> +bool kvm_is_host_reserved_region(u64 gpa) +{ + return
> >>>>>> e820__mbapped_raw_any(gpa-1, gpa+1, E820_TYPE_RESERVED); +}
> >>>>> While _e820__mapped_any()'s doc says '..  checks if any part of
> >>>>> the range <start,end> is mapped ..' it seems to me that the real
> >>>>> check is [start, end) so we should use 'gpa' instead of 'gpa-1',
> >>>>> no?
> >>>> Why do you need to check GPA at all?
> >>>> 
> >>> To reduce the scope of the workaround.
> >>> 
> >>> The errata only happens when you use one of SVM instructions in the
> >>> guest with EAX that happens to be inside one of the host reserved
> >>> memory regions (for example SMM).
> >> 
> >> This code reduces the scope of the workaround at the cost of
> >> increasing the complexity of the workaround and adding a nonsensical
> >> coupling between KVM and host details and adding an export that really
> >> doesn’t deserve to be exported.
> >> 
> >> Is there an actual concrete benefit to this check?
> > 
> > Besides reducing the scope, my intention for the check was that we should
> > know if such exceptions occur for any other undiscovered reasons with other
> > memory types rather than hiding them under this workaround.
> 
> Ask AMD?
> 
> I would also believe that someone somewhere has a firmware that simply omits
> the problematic region instead of listing it as reserved.

I agree with Andy, odds are very good that attempting to be precise will lead to
pain due to false negatives.

And, KVM's SVM instruction emulation needs to be be rock solid regardless of
this behavior since KVM unconditionally intercepts the instruction, i.e. there's
basically zero risk to KVM.