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

[Binbin Wu]

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@xxxxxxxxxxxxxxx>
> >
> > 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)
    that handle differently on them.

    As of this patch, X86EMUL_F_BRANCH and X86EMUL_F_FETCH are 
identical as far as
    KVM is concernered.

    No functional change intended.


> > Signed-off-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
> > Signed-off-by: Zeng Guang <guang.zeng@xxxxxxxxx>
> > ---
> >   arch/x86/kvm/emulate.c     | 5 +++--
> >   arch/x86/kvm/kvm_emulate.h | 1 +
> >   2 files changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > index 3ddfbc99fa4f..8e706d19ae45 100644
> > --- a/arch/x86/kvm/emulate.c
> > +++ b/arch/x86/kvm/emulate.c
> > @@ -721,7 +721,8 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt,
> >   		    (flags & X86EMUL_F_WRITE))
> >   			goto bad;
> >   		/* unreadable code segment */
> > -		if (!(flags & X86EMUL_F_FETCH) && (desc.type & 8) && !(desc.type & 2))
> > +		if (!(flags & (X86EMUL_F_FETCH | X86EMUL_F_BRANCH))
> > +			&& (desc.type & 8) && !(desc.type & 2))
> Put the && on the first line, and align indendation.
I should have been more careful on the alignment & indentation.
Will update it. Thanks.

> 		/* unreadable code segment */
> 		if (!(flags & (X86EMUL_F_FETCH | X86EMUL_F_BRANCH)) &&
> 		    (desc.type & 8) && !(desc.type & 2))
> 			goto bad;