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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by VM instructions
Date:   Tue, 12 Jan 2021 07:51:19 -0800
Message-Id: <8FAC639B-5EC6-42EE-B886-33AEF3CD5E26@amacapital.net>
References: <jpgturmgnu6.fsf@linux.bootlegged.copy>
Cc:     Maxim Levitsky <mlevitsk@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wei Huang <wei.huang2@amd.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, pbonzini@redhat.com,
        seanjc@google.com, joro@8bytes.org, bp@alien8.de,
        tglx@linutronix.de, mingo@redhat.com, x86@kernel.org,
        jmattson@google.com, wanpengli@tencent.com, dgilbert@redhat.com
In-Reply-To: <jpgturmgnu6.fsf@linux.bootlegged.copy>
To:     Bandan Das <bsd@redhat.com>
Precedence: bulk


> On Jan 12, 2021, at 7:46 AM, Bandan Das <bsd@redhat.com> wrote:
>=20
> =EF=BB=BFAndy 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"
>>>>>>=20
>>>>>> 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);
>>>>>>=20
>>>>>> +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?
>>>>=20
>>> To reduce the scope of the workaround.
>>>=20
>>> 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).
>>=20
>> 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=E2=80=99t deserve to be exported.
>>=20
>> Is there an actual concrete benefit to this check?
>=20
> Besides reducing the scope, my intention for the check was that we should
> know if such exceptions occur for any other undiscovered reasons with othe=
r
> 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.

>=20
> Bandan
>=20
>=20
>=20