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Message-ID: <20220831031759.GA130753@k08j02272.eu95sqa>
Date: Wed, 31 Aug 2022 11:17:59 +0800
From: "Hou Wenlong" <houwenlong.hwl@...group.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: x86: Return emulator error if RDMSR/WRMSR
emulation failed
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@...group.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;
> }
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