linux-kernel - Re: [PATCH v2 2/8] KVM: x86: Use a new flag for branch instructions

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Message-ID: <ZNzfgxTnB6KYWENg@google.com>
Date:   Wed, 16 Aug 2023 07:38:59 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Binbin Wu <binbin.wu@...ux.intel.com>
Cc:     Zeng Guang <guang.zeng@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        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>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/8] KVM: x86: Use a new flag for branch instructions

On Wed, Aug 16, 2023, Binbin Wu wrote:
> 
> 
> On 8/16/2023 6:51 AM, Sean Christopherson wrote:
> > Branch *targets*, not branch instructions.
> > 
> > On Wed, Jul 19, 2023, Zeng Guang wrote:
> > > From: Binbin Wu <binbin.wu@...ux.intel.com>
> > > 
> > > Use the new flag X86EMUL_F_BRANCH instead of X86EMUL_F_FETCH in
> > > assign_eip(), since strictly speaking it is not behavior of instruction
> > > fetch.
> > Eh, I'd just drop this paragraph, as evidenced by this code existing as-is for
> > years, we wouldn't introduce X86EMUL_F_BRANCH just because resolving a branch
> > target isn't strictly an instruction fetch.
> > 
> > > Another reason is to distinguish instruction fetch and execution of branch
> > > instruction for feature(s) that handle differently on them.
> > Similar to the shortlog, it's about computing the branch target, not executing a
> > branch instruction.  That distinction matters, e.g. a Jcc that is not taken will
> > *not* follow the branch target, but the instruction is still *executed*.  And there
> > exist instructions that compute branch targets, but aren't what most people would
> > typically consider a branch instruction, e.g. XBEGIN.
> > 
> > > Branch instruction is not data access instruction, so skip checking against
> > > execute-only code segment as instruction fetch.
> > Rather than call out individual use case, I would simply state that as of this
> > patch, X86EMUL_F_BRANCH and X86EMUL_F_FETCH are identical as far as KVM is
> > concernered.  That let's the reader know that (a) there's no intended change in
> > behavior and (b) that the intent is to effectively split all consumption of
> > X86EMUL_F_FETCH into (X86EMUL_F_FETCH | X86EMUL_F_BRANCH).
> 
> How about this:
> 
>     KVM: x86: Use a new flag for branch targets
> 
>     Use the new flag X86EMUL_F_BRANCH instead of X86EMUL_F_FETCH in
> assign_eip()
>     to distinguish instruction fetch and branch target computation for
> feature(s)

Just "features", i.e. no parentheses...

>     that handle differently on them.

...and tack on ", e.g. LASS and LAM." at the end.  There's zero reason not to more
explicitly call out why the flag is being added.  Trying to predict the future in
changelogs is generally discouraged, but having understandable changelogs is more
important.

>     As of this patch, X86EMUL_F_BRANCH and X86EMUL_F_FETCH are identical as
> far as
>     KVM is concernered.
> 
>     No functional change intended.

Heh, you need to fix whatever is forcefully wrapping lines, but other than the
nit above, the content itself is good.