[PATCH] kvm, vmx: don't use "unsigned long" in vmx_vcpu_enter_exit()

[PATCH] kvm, vmx: don't use "unsigned long" in vmx_vcpu_enter_exit()

[Alexey Dobriyan]

__vmx_vcpu_run_flags() returns "unsigned int" and uses only 2 bits of it
so using "unsigned long" is very much pointless.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

 arch/x86/kvm/vmx/vmx.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7067,7 +7067,7 @@ static fastpath_t vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
 
 static noinstr void vmx_vcpu_enter_exit(struct kvm_vcpu *vcpu,
 					struct vcpu_vmx *vmx,
-					unsigned long flags)
+					unsigned int flags)
 {
 	guest_state_enter_irqoff();
 

Re: [PATCH] kvm, vmx: don't use "unsigned long" in vmx_vcpu_enter_exit()

[Sean Christopherson]

Nit (because I really suck at case-insensitive searches), please capitalize
"KVM: VMX:" in the shortlog.

On Fri, Nov 18, 2022, Alexey Dobriyan wrote:
> __vmx_vcpu_run_flags() returns "unsigned int" and uses only 2 bits of it
> so using "unsigned long" is very much pointless.

And __vmx_vcpu_run() and vmx_spec_ctrl_restore_host() take an "unsigned int" as
well, i.e. actually relying on an "unsigned long" value won't actually work.

On a related topic, this code in __vmx_vcpu_run() is unnecessarily fragile as it
relies on VMX_RUN_VMRESUME being in bits 0-7.

	/* Copy @flags to BL, _ASM_ARG3 is volatile. */
	mov %_ASM_ARG3, %bl

	...

	/* Check if vmlaunch or vmresume is needed */
	testb $VMX_RUN_VMRESUME, %bl

The "byte" logic is another holdover, from when "flags" was just "launched" and
was passed in as a boolean.  I'll send a proper patch to do:

diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
index 0b5db4de4d09..5bd39f63497d 100644
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -69,8 +69,8 @@ SYM_FUNC_START(__vmx_vcpu_run)
         */
        push %_ASM_ARG2
 
-       /* Copy @flags to BL, _ASM_ARG3 is volatile. */
-       mov %_ASM_ARG3B, %bl
+       /* Copy @flags to EBX, _ASM_ARG3 is volatile. */
+       mov %_ASM_ARG3L, %ebx
 
        lea (%_ASM_SP), %_ASM_ARG2
        call vmx_update_host_rsp
@@ -106,7 +106,7 @@ SYM_FUNC_START(__vmx_vcpu_run)
        mov (%_ASM_SP), %_ASM_AX
 
        /* Check if vmlaunch or vmresume is needed */
-       testb $VMX_RUN_VMRESUME, %bl
+       test $VMX_RUN_VMRESUME, %ebx
 
        /* Load guest registers.  Don't clobber flags. */
        mov VCPU_RCX(%_ASM_AX), %_ASM_CX
@@ -128,7 +128,7 @@ SYM_FUNC_START(__vmx_vcpu_run)
        /* Load guest RAX.  This kills the @regs pointer! */
        mov VCPU_RAX(%_ASM_AX), %_ASM_AX
 
-       /* Check EFLAGS.ZF from 'testb' above */
+       /* Check EFLAGS.ZF from 'test VMX_RUN_VMRESUME' above */
        jz .Lvmlaunch
 
        /*


> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> ---

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

Re: [PATCH] kvm, vmx: don't use "unsigned long" in vmx_vcpu_enter_exit()

[Sean Christopherson]

On Fri, 18 Nov 2022 20:05:21 +0300, Alexey Dobriyan wrote:
> __vmx_vcpu_run_flags() returns "unsigned int" and uses only 2 bits of it
> so using "unsigned long" is very much pointless.
> 
> 

Applied to kvm-x86 vmx, thanks!

[1/1] kvm, vmx: don't use "unsigned long" in vmx_vcpu_enter_exit()
      https://github.com/kvm-x86/linux/commit/59fc307f5922

--
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