linux-kernel - Re: [PATCH 2/3] KVM: x86: Add support for VMware guest specific hypercalls

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Message-ID: <Z6EeyaOZUevXDBiH@google.com>
Date: Mon, 3 Feb 2025 11:53:45 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: Doug Covelli <doug.covelli@...adcom.com>, Zack Rusin <zack.rusin@...adcom.com>, 
	kvm <kvm@...r.kernel.org>, Jonathan Corbet <corbet@....net>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "the arch/x86 maintainers" <x86@...nel.org>, 
	"H. Peter Anvin" <hpa@...or.com>, Shuah Khan <shuah@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Arnaldo Carvalho de Melo <acme@...hat.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	Joel Stanley <joel@....id.au>, Linux Doc Mailing List <linux-doc@...r.kernel.org>, linux-kernel@...r.kernel.org, 
	linux-kselftest <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH 2/3] KVM: x86: Add support for VMware guest specific hypercalls

On Mon, Feb 03, 2025, Paolo Bonzini wrote:
> On 2/3/25 20:41, Sean Christopherson wrote:
> > -EFAULT isn't the problem, KVM not being able to return useful information in
> > all situations is the issue.
> 
> Yes, that's why I don't want it to be an automatically opted-in API.  If
> incremental improvements are possible, it may be useful to allow interested
> userspace to enable it early.  For example...
> 
> > Specifically, "guest" accesses that are emulated
> > by KVM are problematic, because the -EFAULT from e.g. __kvm_write_guest_page()
> > is disconnected from the code that actually kicks out to userspace.  In that
> > case, userspace will get KVM_EXIT_MMIO, not -EFAULT.  There are more problems
> > beyond KVM_EXIT_MMIO vs. -EFAULT, e.g. instructions that perform multiple memory
> > accesses,
> 
> those are obviously synchronous and I expect VMware to handle them already.
> 
> That said my preferred solution to just use userfaultfd, which is
> synchronous by definition.

Oh, right, userfaultfd would be far better than piggybacking write-tracking.