linux-kernel - Re: [PATCH] KVM/x86: don't use a literal 1 instead of RET_PF

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Message-ID: <Zy6M57VglxCSaZky@google.com>
Date: Fri, 8 Nov 2024 14:12:55 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: "Jürgen Groß" <jgross@...e.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org, x86@...nel.org, 
	kvm@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	"H . Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] KVM/x86: don't use a literal 1 instead of RET_PF_RETRY

On Fri, Nov 08, 2024, Jürgen Groß wrote:
> On 08.11.24 19:44, Sean Christopherson wrote:
> > On Fri, Nov 08, 2024, Paolo Bonzini wrote:
> > > Queued, thanks.
> > 
> > Noooo!  Can you un-queue?
> > 
> > The return from kvm_mmu_page_fault() is NOT RET_PF_xxx, it's KVM outer 0/1/-errno.
> > I.e. '1' is saying "resume the guest", it has *nothing* to do with RET_PF_RETRY.
> > E.g. that path also handles RET_PF_FIXED, RET_PF_SPURIOUS, etc.
> 
> And what about the existing "return RET_PF_RETRY" further up?

Oof.  Works by coincidence.  The intent in that case is to retry the fault, but
the fact that RET_PF_RETRY happens to be '1' is mostly luck.  Returning a postive
value other than '1' should work, but as called out by the comments for the enum,
using '0' for CONTINUE isn't a hard requirement.  E.g. if for some reason we used
'0' for RET_PF_RETRY, this code would break.

 * Note, all values must be greater than or equal to zero so as not to encroach
 * on -errno return values.  Somewhat arbitrarily use '0' for CONTINUE, which
 * will allow for efficient machine code when checking for CONTINUE, e.g.
 * "TEST %rax, %rax, JNZ", as all "stop!" values are non-zero.

FWIW, you are far from the first person to complain about KVM's mostly-undocumented
0/1/-errno return encoding scheme.  The problems is that it's so pervasive
throughout KVM, that in some cases it's not easy to understand if a function is
actually using that scheme, or just happens to return similar values.  I.e.
converting to enums (or #defines) would require a lot of work and churn.