linux-kernel - Re: [PATCH v1 4/5] KVM: Introduce KVM_EXIT

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Message-ID: <Zx_V5SHwzDAl8ZQR@google.com>
Date: Mon, 28 Oct 2024 11:20:21 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Dionna Amalie Glaze <dionnaglaze@...gle.com>
Cc: Binbin Wu <binbin.wu@...ux.intel.com>, Michael Roth <michael.roth@....com>, kvm@...r.kernel.org, 
	linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org, x86@...nel.org, 
	pbonzini@...hat.com, jroedel@...e.de, thomas.lendacky@....com, 
	pgonda@...gle.com, ashish.kalra@....com, bp@...en8.de, pankaj.gupta@....com, 
	liam.merwick@...cle.com, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	Reinette Chatre <reinette.chatre@...el.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	Chao P Peng <chao.p.peng@...el.com>
Subject: Re: [PATCH v1 4/5] KVM: Introduce KVM_EXIT_COCO exit type

On Fri, Sep 13, 2024, Dionna Amalie Glaze wrote:
> We can extend the ccp driver to, on extended guest request, lock the
> command buffer, get the REPORTED_TCB, complete the request, unlock the
> command buffer, and return both the response and the REPORTED_TCB at
> the time of the request. 

Holding a lock across an exit to userspace seems wildly unsafe.

Can you explain the race that you are trying to close, with the exact "bad" sequence
of events laid out in chronological order, and an explanation of why the race can't
be sovled in userspace?  I read through your previous comment[*] (which I assume
is the race you want to close?), but I couldn't quite piece together exactly what's
broken.

[*] https://lore.kernel.org/all/CAAH4kHb03Una2kcvyC3W=1ZfANBWF_7a7zsSmWhr_r9g3rCDZw@mail.gmail.com