x86/virt: Mark flags and memory as clobbered by VMXOFF
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 4b1e5478 ("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: NDavid P. Reed <dpreed@deepplum.com> [sean: rewrote changelog, dropped comment adjustments] Signed-off-by: NSean Christopherson <seanjc@google.com> Message-Id: <20201231002702.22237077-4-seanjc@google.com> Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
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