linux-kernel - Re: [PATCH RFCv2 9/9] arm64: Support async page fault

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Date:   Fri, 29 May 2020 10:41:44 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Gavin Shan <gshan@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, shan.gavin@...il.com,
        catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH RFCv2 9/9] arm64: Support async page fault

On 2020-05-29 00:02, Gavin Shan wrote:
> Hi Paolo,
> 
> On 5/28/20 8:48 PM, Paolo Bonzini wrote:
>> On 28/05/20 08:14, Gavin Shan wrote:
>>>> - for x86 we're also thinking of initiating the page fault from the
>>>> exception handler, rather than doing so from the hypervisor before
>>>> injecting the exception.  If ARM leads the way here, we would do our
>>>> best to share code when x86 does the same.
>>> 
>>> Sorry, Paolo, I don't follow your idea here. Could you please provide
>>> more details?
>> 
>> The idea is to inject stage2 page faults into the guest even before 
>> the
>> host starts populates the page.  The guest then invokes a hypercall,
>> telling the host to populate the page table and inject the 'page 
>> ready'
>> event (interrupt) when it's done.
>> 
>> For x86 the advantage is that the processor can take care of raising 
>> the
>> stage2 page fault in the guest, so it's faster.
>> 
> I think there might be too much overhead if the page can be populated
> quickly by host. For example, it's fast to populate the pages if swapin
> isn't involved.
> 
> If I'm correct enough, it seems arm64 doesn't have similar mechanism,
> routing stage2 page fault to guest.

Indeed, this isn't a thing on arm64. Exception caused by a S2 fault are
always routed to EL2.

         M.
-- 
Jazz is not dead. It just smells funny...