linux-kernel - Re: [PATCH RFC 4/6] KVM: x86: acknowledgment mechanism for async pf page ready notifications

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Date:   Thu, 30 Apr 2020 08:46:18 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, X86 ML <x86@...nel.org>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH RFC 4/6] KVM: x86: acknowledgment mechanism for async pf
 page ready notifications

On 30/04/20 02:45, Andy Lutomirski wrote:
>> That's a very bad idea since one is synchronous and one is asynchronous.
>>  Part of the proposal we agreed upon was to keep "page not ready"
>> synchronous while making "page ready" an interrupt.  The data structure
>> for "page not ready" will be #VE.
>
> #VE on SVM will be interesting, to say the least, and I think that a
> solution that is VMX specific doesn't make much sense.

You can always inject it manually.  The same is true of Haswell and
earlier processors.

> #VE also has
> unpleasant issues involving the contexts in which it can occur.  You
> will have quite a hard time convincing me to ack the addition of a #VE
> entry handler for this.  I think a brand new vector is the right
> solution.

I need --verbose. :)  For #VE I liked the idea of re-enabling it from an
IPI, at least in the case where we cannot move out of the IST stack.
And any other vector that behaves like an exception would have the same
issue, wouldn't it (especially re-entrancy)?

Paolo