linux-kernel - Re: [PATCH v2 1/3] kvm: svm: Add support for additional SVM NPF error codes

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Date:   Fri, 4 Aug 2017 16:05:45 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     kvm@...r.kernel.org, thomas lendacky <thomas.lendacky@....com>,
        rkrcmar@...hat.com, joro@...tes.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
        tglx@...utronix.de, bp@...e.de
Subject: Re: [PATCH v2 1/3] kvm: svm: Add support for additional SVM NPF error
 codes

On 04/08/2017 02:30, Brijesh Singh wrote:
> 
> 
> On 8/2/17 5:42 AM, Paolo Bonzini wrote:
>> On 01/08/2017 15:36, Brijesh Singh wrote:
>>>> The flow is:
>>>>
>>>>    hardware walks page table; L2 page table points to read only memory
>>>>    -> pf_interception (code =
>>>>    -> kvm_handle_page_fault (need_unprotect = false)
>>>>    -> kvm_mmu_page_fault
>>>>    -> paging64_page_fault (for example)
>>>>       -> try_async_pf
>>>>          map_writable set to false
>>>>       -> paging64_fetch(write_fault = true, map_writable = false,
>>>> prefault = false)
>>>>          -> mmu_set_spte(speculative = false, host_writable = false,
>>>> write_fault = true)
>>>>             -> set_spte
>>>>                mmu_need_write_protect returns true
>>>>                return true
>>>>             write_fault == true -> set emulate = true
>>>>             return true
>>>>          return true
>>>>       return true
>>>>    emulate
>>>>
>>>> Without this patch, emulation would have called
>>>>
>>>>    ..._gva_to_gpa_nested
>>>>    -> translate_nested_gpa
>>>>    -> paging64_gva_to_gpa
>>>>    -> paging64_walk_addr
>>>>    -> paging64_walk_addr_generic
>>>>       set fault (nested_page_fault=true)
>>>>
>>>> and then:
>>>>
>>>>     kvm_propagate_fault
>>>>     -> nested_svm_inject_npf_exit
>>>>
>>> maybe then safer thing would be to qualify the new error_code check with
>>> !mmu_is_nested(vcpu) or something like that. So that way it would run on
>>> L1 guest, and not the L2 guest. I believe that would restrict it avoid
>>> hitting this case. Are you okay with this change ?
>> Or check "vcpu->arch.mmu.direct_map"?  That would be true when not using
>> shadow pages.
> 
> Yes that can be used.

Are you going to send a patch for this?

Paolo

>>> IIRC, the main place where this check was valuable was when L1 guest had
>>> a fault (when coming out of the L2 guest) and emulation was not needed.
>> How do I measure the effect?  I tried counting the number of emulations,
>> and any difference from the patch was lost in noise.
> 
> I think this patch is necessary for functional reasons (not just
> perf), because we added the other patch to look at the GPA and stop
> walking the guest page tables on a NPF.
> 
> The issue I think was that hardware has taken an NPF because the page
> table is marked RO, and it saves the GPA in the VMCB. KVM was then going
> and emulating the instruction and it saw that a GPA was available. But
> that GPA was not the GPA of the instruction it is emulating, since it
> was the GPA of the tablewalk page that had the fault. It was debugged
> that at the time and realized that emulating the instruction was
> unnecessary so we added this new code in there which fixed the
> functional issue and helps perf.
> 
> I don't have any data on how much perf, as I recall it was most
> effective when the L1 guest page tables and L2 nested page tables were
> exactly the same. In that case, it avoided emulations for code that L1
> executes which I think could be as much as one emulation per 4kb code page.
>