Re: [PATCH 3/5] KVM: VMX: Ensure interruptibility when single-stepping

[Avi Kivity <avi@redhat.com>]

Jan Kiszka wrote:
> When single-stepping, we have to ensure that the INT1 can make it
> through even if the guest itself is uninterruptible due to MOV SS or
> STI. VMENTRY will fail otherwise.
>
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
>
>  arch/x86/kvm/vmx.c |   10 ++++++++--
>  1 files changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 3a422dc..8e83102 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -1010,6 +1010,7 @@ static void vmx_cache_reg(struct kvm_vcpu *vcpu, enum kvm_reg reg)
>  static int set_guest_debug(struct kvm_vcpu *vcpu, struct kvm_guest_debug *dbg)
>  {
>  	int old_debug = vcpu->guest_debug;
> +	u32 interruptibility;
>  	unsigned long flags;
>  
>  	vcpu->guest_debug = dbg->control;
> @@ -1017,9 +1018,14 @@ static int set_guest_debug(struct kvm_vcpu *vcpu, struct kvm_guest_debug *dbg)
>  		vcpu->guest_debug = 0;
>  
>  	flags = vmcs_readl(GUEST_RFLAGS);
> -	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
> +	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
>  		flags |= X86_EFLAGS_TF | X86_EFLAGS_RF;
> -	else if (old_debug & KVM_GUESTDBG_SINGLESTEP)
> +		/* We must be interruptible when single-stepping */
> +		interruptibility = vmcs_read32(GUEST_INTERRUPTIBILITY_INFO);
> +		if (interruptibility & 3)
> +			vmcs_write32(GUEST_INTERRUPTIBILITY_INFO,
> +				     interruptibility & ~3);
>   

Could just write unconditionally - it's not like the write has any 
effect on speed.  vmcs_clear_bits() will do it cleanly.

But I'm worried about correctness.  Suppose we're singlestepping a sti; 
hlt sequence.  While we'll clear interruptibility, probably receive the 
debug trap (since that's a high priority exception), but then inject the 
interrupt before the hlt, hanging the guest.  So we probably need to 
restore interruptibility on exit.

This looks like a good candidate for a test case.

-- 
error compiling committee.c: too many arguments to function