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[v3,02/33] KVM: nVMX: Check a single byte for VMCS "launched" in nested early checks

Message ID 20190125154120.19385-3-sean.j.christopherson@intel.com (mailing list archive)
State New, archived
Headers show
Series KVM: VMX: Move vCPU-run to proper asm sub-routine | expand

Commit Message

Sean Christopherson Jan. 25, 2019, 3:40 p.m. UTC 
Nested early checks does a manual comparison of a VMCS' launched status
in its asm blob to execute the correct VM-Enter instruction, i.e.
VMLAUNCH vs. VMRESUME.  The launched flag is a bool, which is a typedef
of _Bool.  C99 does not define an exact size for _Bool, stating only
that is must be large enough to hold '0' and '1'.  Most, if not all,
compilers use a single byte for _Bool, including gcc[1].

The use of 'cmpl' instead of 'cmpb' was not deliberate, but rather the
result of a copy-paste as the asm blob was directly derived from the asm
blob for vCPU-run.

This has not caused any known problems, likely due to compilers aligning
variables to 4-byte or 8-byte boundaries and KVM zeroing out struct
vcpu_vmx during allocation.  I.e. vCPU-run accesses "junk" data, it just
happens to always be zero and so doesn't affect the result.

[1] https://gcc.gnu.org/ml/gcc-patches/2000-10/msg01127.html

Fixes: 52017608da33 ("KVM: nVMX: add option to perform early consistency checks via H/W")
Cc: <stable@vger.kernel.org>
Reviewed-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
 arch/x86/kvm/vmx/nested.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
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Patch

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 2616bd2c7f2c..cfcf67e548fa 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -2760,7 +2760,7 @@  static int nested_vmx_check_vmentry_hw(struct kvm_vcpu *vcpu)
 		"add $%c[wordsize], %%" _ASM_SP "\n\t" /* un-adjust RSP */
 
 		/* Check if vmlaunch or vmresume is needed */
-		"cmpl $0, %c[launched](%% " _ASM_CX")\n\t"
+		"cmpb $0, %c[launched](%% " _ASM_CX")\n\t"
 
 		"call vmx_vmenter\n\t"