linux-kernel - Re: [PATCH 0/4] KVM: fix KVM_EXIT_SYSTEM

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Message-ID: <36978590-c714-d0ba-6e4e-ab0016136381@redhat.com>
Date:   Fri, 22 Apr 2022 12:19:51 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Oliver Upton <oupton@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, will@...nel.org, apatel@...tanamicro.com,
        atishp@...osinc.com, seanjc@...gle.com, pgonda@...gle.com
Subject: Re: [PATCH 0/4] KVM: fix KVM_EXIT_SYSTEM_EVENT mess

On 4/22/22 12:01, Marc Zyngier wrote:
>> These patches are against kvm/next, so that is already what I did. :)
> 
> Can you please post a complete series? It is becoming really hard to
> track what you are doing.

Yes, I'll post a complete series for kvm/master in a second.

>> On the other hand right now the ARM and RISC-V flags are unusable with
>> 32-bit userspace, so we need to fix _something_ in 5.18 as well.
> 
> What 32bit userspace? arm64 doesn't have any that can interact with KVM,
> so I don't see anything to fix on that front.

You're right, ARM64 does not define KVM_COMPAT.  But RISC-V does.

>> For your proposal, all that's missing is a 5.18 patch to add the
>> padding. But since the flags UAPI was completely unused before 5.18
>> and there's no reason to inflict the different naming of fields to
>> userspace.  So I think we want to apply this UAPI change in 5.18 too.
> 
> As it was pointed out already, CrosVM has already started looking at
> the flags. The fact that it was always 0 until now doesn't make it
> less of a UAPI.

I heard that and that's exactly why I dropped the idea of using 
NDATA_VALID in bit 31 of flags, and switched to a capability instead. 
If it is desirable for Android, crosvm can "quirk" that ARM always has 
valid data[0].

Paolo

> I'd like to see a full series that implements the transition before we
> make a decision on this.