From: "David P. Reed" <dpreed@deepplum.com>

Date: Wed, 30 Dec 2020 16:26:56 -0800

Subject: x86/virt: Mark flags and memory as clobbered by VMXOFF

Git-commit: 53666664a3052e4ea3ddcb183460dfbc30f1d056

Patch-mainline: v5.12-rc1

References: git-fixes

Explicitly tell the compiler that VMXOFF modifies flags (like all VMX

instructions), and mark memory as clobbered since VMXOFF must not be

reordered and also may have memory side effects (though the kernel

really shouldn't be accessing the root VMCS anyways).

Practically speaking, adding the clobbers is most likely a nop; the

primary motivation is to properly document VMXOFF's behavior.

For the flags clobber, both Clang and GCC automatically mark flags as

clobbered; this is noted in commit 4b1e54786e48 ("KVM/x86: Use assembly

instruction mnemonics instead of .byte streams"), which intentionally

removed the previous clobber.  But, neither Clang nor GCC documents

this behavior, and there's no downside to including the clobber.

For the memory clobber, the RFLAGS.IF and CR4.VMXE manipulations that

immediately follow VMXOFF have compiler barriers of their own, i.e.

VMXOFF can't get reordered after clearing CR4.VMXE, which is really

what's of interest.

Cc: Randy Dunlap <rdunlap@infradead.org>

Signed-off-by: David P. Reed <dpreed@deepplum.com>

[sean: rewrote changelog, dropped comment adjustments]

Signed-off-by: Sean Christopherson <seanjc@google.com>

Message-Id: <20201231002702.2223707-4-seanjc@google.com>

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Acked-by: Nikolay Borisov <nik.borisov@suse.com>

---

 arch/x86/include/asm/virtext.h | 3 ++-

 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/virtext.h b/arch/x86/include/asm/virtext.h

index fda3e7747c22..2cc585467667 100644

--- a/arch/x86/include/asm/virtext.h

+++ b/arch/x86/include/asm/virtext.h

@@ -44,7 +44,8 @@ static inline int cpu_has_vmx(void)

 static inline void cpu_vmxoff(void)

 {

 	asm_volatile_goto("1: vmxoff\n\t"

-			  _ASM_EXTABLE(1b, %l[fault]) :::: fault);

+			  _ASM_EXTABLE(1b, %l[fault])

+			  ::: "cc", "memory" : fault);

 fault:

 	cr4_clear_bits(X86_CR4_VMXE);

 }