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SubjectRe: [PATCH v2 2/8] KVM: x86: Use a new flag for branch instructions
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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@linux.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)
    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@linux.intel.com>
>> Signed-off-by: Zeng Guang <guang.zeng@intel.com>
>> ---
>> 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;

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