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Date:   Tue, 12 Jan 2021 07:51:19 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Bandan Das <bsd@...hat.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wei Huang <wei.huang2@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, pbonzini@...hat.com,
        seanjc@...gle.com, joro@...tes.org, bp@...en8.de,
        tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
        jmattson@...gle.com, wanpengli@...cent.com, dgilbert@...hat.com
Subject: Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by VM instructions


> On Jan 12, 2021, at 7:46 AM, Bandan Das <bsd@...hat.com> wrote:
> 
> Andy Lutomirski <luto@...capital.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.

> 
> Bandan
> 
> 
> 

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