Re: [PATCH 1/2] KVM: x86: Return emulator error if RDMSR/WRMSR emulation failed

["Hou Wenlong" <houwenlong.hwl@antgroup.com>]

On Wed, Aug 31, 2022 at 02:44:08AM +0800, Sean Christopherson wrote:
> On Thu, Jul 28, 2022, Hou Wenlong wrote:
> > The return value of emulator_{get|set}_mst_with_filter()
> > is confused, since msr access error and emulator error
> > are mixed. Although, KVM_MSR_RET_* doesn't conflict with
> > X86EMUL_IO_NEEDED at present, it is better to convert
> > msr access error to emulator error if error value is
> > needed.
> > 
> > Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
> > ---
> >  arch/x86/kvm/x86.c | 22 ++++++++++++----------
> >  1 file changed, 12 insertions(+), 10 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 5366f884e9a7..8df89b9c212f 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -7908,11 +7908,12 @@ static int emulator_get_msr_with_filter(struct x86_emulate_ctxt *ctxt,
> >  	int r;
> >  
> >  	r = kvm_get_msr_with_filter(vcpu, msr_index, pdata);
> > -
> > -	if (r && kvm_msr_user_space(vcpu, msr_index, KVM_EXIT_X86_RDMSR, 0,
> > -				    complete_emulated_rdmsr, r)) {
> > -		/* Bounce to user space */
> > -		return X86EMUL_IO_NEEDED;
> > +	if (r) {
> > +		if (kvm_msr_user_space(vcpu, msr_index, KVM_EXIT_X86_RDMSR, 0,
> > +				       complete_emulated_rdmsr, r))
> > +			r = X86EMUL_IO_NEEDED;
> > +		else
> > +			r = X86EMUL_UNHANDLEABLE;
> 
> This should be X86EMUL_PROPAGATE_FAULT, X86EMUL_UNHANDLEABLE is used to indicate
> that KVM needs to bail all the way to userspace.
> 
> I definitely like the idea of converting to X86EMUL_* here instead of spreading
> it across these helpers and the emulator, but in that case should convert _all_
> types.
> 
> And I think it makes sense to opportunistically handle "r < 0" in the get helper.
> KVM may not return -errno today, but assuming that will always hold true is
> unnecessarily risking.
I agree. The original commit 7dffecaf4eab wanted to report negative values to
userspace, but the emulator actually didn't propagate -errno to the caller.
So handling "r < 0" in the set helper is better, then only X86EMUL_* is returned. 

> 
> E.g. what about:
> 
> 
> static int emulator_get_msr_with_filter(struct x86_emulate_ctxt *ctxt,
> 					u32 msr_index, u64 *pdata)
> {
> 	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
> 	int r;
> 
> 	r = kvm_get_msr_with_filter(vcpu, msr_index, pdata);
> 	if (r < 0)
> 		return X86EMUL_UNHANDLEABLE;
> 
> 	if (r) {
> 		if (kvm_msr_user_space(vcpu, msr_index, KVM_EXIT_X86_RDMSR, 0,
> 				       complete_emulated_rdmsr, r))
> 			return X86EMUL_IO_NEEDED;
> 		else
> 			return X86EMUL_PROPAGATE_FAULT;
> 	}
> 
> 	return X86EMUL_CONTINUE;
> }
> 
> static int emulator_set_msr_with_filter(struct x86_emulate_ctxt *ctxt,
> 					u32 msr_index, u64 data)
> {
> 	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
> 	int r;
> 
> 	r = kvm_set_msr_with_filter(vcpu, msr_index, data);
> 	if (r < 0)
> 		return X86EMUL_UNHANDLEABLE;
> 
> 	if (r) {
> 		if (kvm_msr_user_space(vcpu, msr_index, KVM_EXIT_X86_WRMSR, data,
> 				       complete_emulated_msr_access, r))
> 			return X86EMUL_IO_NEEDED;
> 		else
> 			return X86EMUL_PROPAGATE_FAULT;
> 	}
> 
> 	return X86EMUL_CONTINUE;
> }
>
I'll take this in the v2. Thanks.

> 
> Or maybe even add a helper to do the translation?  Can't tell if this is a net
> positive or not.  It's a bit gratuitous, but it does ensure consistent behavior
> for RDMSR vs. WRMSR.
> 
> static int emulator_handle_msr_return(struct kvm_vcpu *vcpu *, int r,
> 				      u32 msr, u64 data, u32 exit_reason,
> 				      int (*comp)(struct kvm_vcpu *vcpu))
> {
> 	if (r < 0)
> 		return X86EMUL_UNHANDLEABLE;
> 
> 	if (r) {
> 		if (kvm_msr_user_space(vcpu, msr, exit_reason, data, comp, r))
> 			return X86EMUL_IO_NEEDED;
> 		else
> 			return X86EMUL_UNHANDLEABLE;
> 	}
> 
> 	return X86EMUL_CONTINUE;
> }
> 
> static int emulator_get_msr_with_filter(struct x86_emulate_ctxt *ctxt,
> 					u32 msr_index, u64 *pdata)
> {
> 	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
> 	int r;
> 
> 	r = kvm_get_msr_with_filter(vcpu, msr_index, pdata);
> 	return emulator_handle_msr_return(vcpu, r, msr_index, 0,
> 					  KVM_EXIT_X86_RDMSR,
> 					  complete_emulated_rdmsr);
> }
> 
> static int emulator_set_msr_with_filter(struct x86_emulate_ctxt *ctxt,
> 					u32 msr_index, u64 data)
> {
> 	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
> 	int r;
> 
> 	r = kvm_set_msr_with_filter(vcpu, msr_index, data);
> 	return emulator_handle_msr_return(vcpu, r, msr_index, data,
> 					  KVM_EXIT_X86_WRMSR,
> 					  complete_emulated_msr_access);
> }
> 
> 
> And then the emulator side of things can be:
> 
> static int em_wrmsr(struct x86_emulate_ctxt *ctxt)
> {
> 	u64 msr_index = reg_read(ctxt, VCPU_REGS_RCX);
> 	u64 msr_data;
> 	int r;
> 
> 	msr_data = (u32)reg_read(ctxt, VCPU_REGS_RAX)
> 		| ((u64)reg_read(ctxt, VCPU_REGS_RDX) << 32);
> 	r = ctxt->ops->set_msr_with_filter(ctxt, msr_index, msr_data);
> 
> 	if (r == X86EMUL_PROPAGATE_FAULT)
> 		return emulate_gp(ctxt, 0);
> 
> 	return r;
> }
> 
> static int em_rdmsr(struct x86_emulate_ctxt *ctxt)
> {
> 	u64 msr_index = reg_read(ctxt, VCPU_REGS_RCX);
> 	u64 msr_data;
> 	int r;
> 
> 	r = ctxt->ops->get_msr_with_filter(ctxt, msr_index, &msr_data);
> 
> 	if (r == X86EMUL_PROPAGATE_FAULT)
> 		return emulate_gp(ctxt, 0);
> 
> 	if (r == X86EMUL_CONTINUE) {
> 		*reg_write(ctxt, VCPU_REGS_RAX) = (u32)msr_data;
> 		*reg_write(ctxt, VCPU_REGS_RDX) = msr_data >> 32;
> 	}
> 	return r;
> }