Can you use "KVM: x86" for the scope? "x86/kvm" is used for guest changes, i.e.
for paravirt code when running as a KVM guest.
On Mon, Nov 11, 2024, Peter Zijlstra wrote:
> Current test_cc() uses the fastop infrastructure to test flags using
> SETcc instructions. However, int3_emulate_jcc() already fully
> implements the flags->CC mapping, use that.
I think it's worth presenting this as a revert of sorts, even though it's not a
strict revert. KVM also emulated jcc-like operations in software prior to commit
9ae9febae950 ("KVM: x86 emulator: covert SETCC to fastop"), i.e. the fastop
madness was introduced for performance reasons, not because writing the code was
hard.
Looking at the output of the fastop versus __emulate_cc(), the worst case cost is
two extra conditional branches. Assuming that eliminating the test_cc() fastop
code avoids having to add another special case in objtool, I am a-ok with the
tradeoff. Especially since emulating instructions that use test_cc() is largely
limited to older hardware running guest firmware that uses (emulated) SMM, and
maybe a few bespoke use cases. E.g. I get literally zero hits on test_cc() when
booting a 24 vCPU VM (64-bit or 32-bit kernel) with EPT and unrestricted guest
disabled, as the OVMF build I use doesn't rely on SMM.
And FWIW, a straight revert appears to generate worse code. I see no reason to
bring it back.
With a massaged shortlog+changelog,
Acked-by: Sean Christopherson <seanjc@xxxxxxxxxx>
fastop:
0x0000000000042c41 <+1537>: movzbl 0x61(%rbp),%eax
0x0000000000042c45 <+1541>: mov 0x10(%rbp),%rdx
0x0000000000042c49 <+1545>: shl $0x4,%rax
0x0000000000042c4d <+1549>: and $0xf0,%eax
0x0000000000042c52 <+1554>: and $0x8d5,%edx
0x0000000000042c58 <+1560>: or $0x2,%dh
0x0000000000042c5b <+1563>: add $0x0,%rax
0x0000000000042c61 <+1569>: push %rdx
0x0000000000042c62 <+1570>: popf
0x0000000000042c63 <+1571>: call 0x42c68 <x86_emulate_insn+1576>
0x0000000000042c68 <+1576>: mov %eax,%edx
0x0000000000042c6a <+1578>: xor %eax,%eax
0x0000000000042c6c <+1580>: test %dl,%dl
0x0000000000042c6e <+1582>: jne 0x42f24 <x86_emulate_insn+2276>
__emulate_cc:
0x0000000000042b95 <+1541>: movzbl 0x61(%rbp),%eax
0x0000000000042b99 <+1545>: mov 0x10(%rbp),%rcx
0x0000000000042b9d <+1549>: and $0xf,%eax
0x0000000000042ba0 <+1552>: cmp $0xb,%al
0x0000000000042ba2 <+1554>: ja 0x42e90 <x86_emulate_insn+2304>
0x0000000000042e90 <+2304>: mov %rcx,%rdx
0x0000000000042e93 <+2307>: mov %rcx,%rsi
0x0000000000042e96 <+2310>: shr $0x7,%rdx
0x0000000000042e9a <+2314>: shr $0xb,%rsi
0x0000000000042e9e <+2318>: xor %rsi,%rdx
0x0000000000042ea1 <+2321>: cmp $0xd,%al
0x0000000000042ea3 <+2323>: ja 0x4339a <x86_emulate_insn+3594>
0x000000000004339a <+3594>: and $0x1,%edx
0x000000000004339d <+3597>: and $0x40,%ecx
0x00000000000433a0 <+3600>: or %rcx,%rdx
0x00000000000433a3 <+3603>: setne %dl
0x00000000000433a6 <+3606>: jmp 0x42bba <x86_emulate_insn+1578>
0x0000000000042ea9 <+2329>: and $0x1,%edx
0x0000000000042eac <+2332>: jmp 0x42bba <x86_emulate_insn+1578>
0x0000000000042ba8 <+1560>: mov %eax,%edx
0x0000000000042baa <+1562>: shr %dl
0x0000000000042bac <+1564>: and $0x7,%edx
0x0000000000042baf <+1567>: and 0x0(,%rdx,8),%rcx
0x0000000000042bb7 <+1575>: setne %dl
0x0000000000042bba <+1578>: test $0x1,%al
0x0000000000042bbc <+1580>: jne 0x43323 <x86_emulate_insn+3475>
0x0000000000043323 <+3475>: test %dl,%dl
0x0000000000043325 <+3477>: jne 0x4332f <x86_emulate_insn+3487>
0x0000000000043327 <+3479>: test $0x1,%al
0x0000000000043329 <+3481>: jne 0x42bca <x86_emulate_insn+1594>
0x000000000004332f <+3487>: xor %eax,%eax
0x0000000000043331 <+3489>: jmp 0x42be0 <x86_emulate_insn+1616>
0x0000000000042bc2 <+1586>: test %dl,%dl
0x0000000000042bc4 <+1588>: je 0x43327 <x86_emulate_insn+3479>
0x0000000000043327 <+3479>: test $0x1,%al
0x0000000000043329 <+3481>: jne 0x42bca <x86_emulate_insn+1594>
0x000000000004332f <+3487>: xor %eax,%eax
0x0000000000043331 <+3489>: jmp 0x42be0 <x86_emulate_insn+1616>
0x0000000000042bca <+1594>: movslq 0xd0(%rbp),%rsi
0x0000000000042bd1 <+1601>: mov %rbp,%rdi
0x0000000000042bd4 <+1604>: add 0x90(%rbp),%rsi
0x0000000000042bdb <+1611>: call 0x3a7c0 <assign_eip>
revert:
0x0000000000042bc9 <+1545>: movzbl 0x61(%rbp),%esi
0x0000000000042bcd <+1549>: mov 0x10(%rbp),%rdx
0x0000000000042bd1 <+1553>: mov %esi,%eax
0x0000000000042bd3 <+1555>: shr %eax
0x0000000000042bd5 <+1557>: and $0x7,%eax
0x0000000000042bd8 <+1560>: cmp $0x4,%eax
0x0000000000042bdb <+1563>: je 0x42ed4 <x86_emulate_insn+2324>
0x0000000000042ed4 <+2324>: mov %edx,%ecx
0x0000000000042ed6 <+2326>: and $0x80,%ecx
0x0000000000042edc <+2332>: jmp 0x42bff <x86_emulate_insn+1599>
0x0000000000042be1 <+1569>: ja 0x43225 <x86_emulate_insn+3173>
0x0000000000043225 <+3173>: cmp $0x6,%eax
0x0000000000043228 <+3176>: je 0x434ec <x86_emulate_insn+3884>
0x00000000000434ec <+3884>: xor %edi,%edi
0x00000000000434ee <+3886>: jmp 0x43238 <x86_emulate_insn+3192>
0x000000000004322e <+3182>: mov %edx,%edi
0x0000000000043230 <+3184>: and $0x40,%edi
0x0000000000043233 <+3187>: cmp $0x7,%eax
0x0000000000043236 <+3190>: jne 0x4325c <x86_emulate_insn+3228>
0x000000000004325c <+3228>: mov %edx,%ecx
0x000000000004325e <+3230>: and $0x4,%ecx
0x0000000000043261 <+3233>: cmp $0x5,%eax
0x0000000000043264 <+3236>: je 0x42bff <x86_emulate_insn+1599>
0x000000000004326a <+3242>: jmp 0x43218 <x86_emulate_insn+3160>
0x0000000000043238 <+3192>: mov %rdx,%rcx
0x000000000004323b <+3195>: shr $0xb,%rdx
0x000000000004323f <+3199>: shr $0x7,%rcx
0x0000000000043243 <+3203>: xor %edx,%ecx
0x0000000000043245 <+3205>: and $0x1,%ecx
0x0000000000043248 <+3208>: or %edi,%ecx
0x000000000004324a <+3210>: jmp 0x42bff <x86_emulate_insn+1599>
0x0000000000042be7 <+1575>: mov %edx,%ecx
0x0000000000042be9 <+1577>: and $0x40,%ecx
0x0000000000042bec <+1580>: cmp $0x2,%eax
0x0000000000042bef <+1583>: je 0x42bff <x86_emulate_insn+1599>
0x0000000000042bf1 <+1585>: mov %edx,%ecx
0x0000000000042bf3 <+1587>: and $0x41,%ecx
0x0000000000042bf6 <+1590>: cmp $0x3,%eax
0x0000000000042bf9 <+1593>: jne 0x4320a <x86_emulate_insn+3146>
0x000000000004320a <+3146>: mov %edx,%ecx
0x000000000004320c <+3148>: and $0x1,%ecx
0x000000000004320f <+3151>: cmp $0x1,%eax
0x0000000000043212 <+3154>: je 0x42bff <x86_emulate_insn+1599>
0x0000000000043218 <+3160>: mov %edx,%ecx
0x000000000004321a <+3162>: and $0x800,%ecx
0x0000000000043220 <+3168>: jmp 0x42bff <x86_emulate_insn+1599>
0x0000000000042bff <+1599>: test %ecx,%ecx
0x0000000000042c01 <+1601>: setne %dl
0x0000000000042c04 <+1604>: and $0x1,%esi
0x0000000000042c07 <+1607>: xor %eax,%eax
0x0000000000042c09 <+1609>: cmp %sil,%dl
0x0000000000042c0c <+1612>: je 0x42c24 <x86_emulate_insn+1636>
0x0000000000042c0e <+1614>: movslq 0xd0(%rbp),%rsi
0x0000000000042c15 <+1621>: mov %rbp,%rdi
0x0000000000042c18 <+1624>: add 0x90(%rbp),%rsi
0x0000000000042c1f <+1631>: call 0x3a7c0 <assign_eip>
