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Date: Thu, 2 Sep 2021 17:34:50 +0000
From: Sean Christopherson <seanjc@...gle.com>
To: Maxim Levitsky <mlevitsk@...hat.com>
Cc: kvm@...r.kernel.org, Kieran Bingham <kbingham@...nel.org>,
Jan Kiszka <jan.kiszka@...mens.com>,
Andrew Jones <drjones@...hat.com>,
Jonathan Corbet <corbet@....net>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Ingo Molnar <mingo@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
Johannes Berg <johannes.berg@...el.com>,
Wanpeng Li <wanpengli@...cent.com>,
"H. Peter Anvin" <hpa@...or.com>, Jessica Yu <jeyu@...nel.org>,
Jim Mattson <jmattson@...gle.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Joerg Roedel <joro@...tes.org>,
Yang Weijiang <weijiang.yang@...el.com>,
linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
"open list:KERNEL SELFTEST FRAMEWORK"
<linux-kselftest@...r.kernel.org>,
"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
Shuah Khan <shuah@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 3/6] KVM: SVM: implement
force_intercept_exceptions_mask
On Wed, Aug 11, 2021, Maxim Levitsky wrote:
> On Wed, 2021-08-11 at 15:29 +0300, Maxim Levitsky wrote:
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index e45259177009..19f54b07161a 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -233,6 +233,8 @@ static const u32 msrpm_ranges[] = {0, 0xc0000000, 0xc0010000};
> > #define MSRS_RANGE_SIZE 2048
> > #define MSRS_IN_RANGE (MSRS_RANGE_SIZE * 8 / 2)
> >
> > +static int svm_handle_invalid_exit(struct kvm_vcpu *vcpu, u64 exit_code);
> > +
> > u32 svm_msrpm_offset(u32 msr)
> > {
> > u32 offset;
> > @@ -1153,6 +1155,22 @@ static void svm_recalc_instruction_intercepts(struct kvm_vcpu *vcpu,
> > }
> > }
> >
> > +static void svm_init_force_exceptions_intercepts(struct vcpu_svm *svm)
> > +{
> > + int exc;
> > +
> > + svm->force_intercept_exceptions_mask = force_intercept_exceptions_mask;
Ah, the param is being snapshotted on vCPU creation, hence the writable module
param. That works, though it'd be better to snapshot it on a per-VM basic, not
per-vCPU, and do so in common x86 code so that the param doesn't need to be
exported.
> > + for (exc = 0 ; exc < 32 ; exc++) {
for_each_set_bit()
> > + if (!(svm->force_intercept_exceptions_mask & (1 << exc)))
> > + continue;
> > +
> > + /* Those are defined to have undefined behavior in the SVM spec */
> > + if (exc != 2 && exc != 9)
Maybe add a pr_warn_once() to let the user know they done messed up?
And given that there are already intercepts with undefined behavior, it's probably
best to disallow intercepting anything we aren't 100% postive will be handled
correctly, e.g. intercepting vector 31 is nonsensical at this time.
> > + continue;
> > + set_exception_intercept(svm, exc);
...
> > +static int gen_exc_interception(struct kvm_vcpu *vcpu)
> > +{
> > + /*
> > + * Generic exception intercept handler which forwards a guest exception
> > + * as-is to the guest.
> > + * For exceptions that don't have a special intercept handler.
> > + *
> > + * Used only for 'force_intercept_exceptions_mask' KVM debug feature.
> > + */
> > + struct vcpu_svm *svm = to_svm(vcpu);
> > + int exc = svm->vmcb->control.exit_code - SVM_EXIT_EXCP_BASE;
> > +
> > + /* SVM doesn't provide us with an error code for the #DF */
> > + u32 err_code = exc == DF_VECTOR ? 0 : svm->vmcb->control.exit_info_1;
Might be better to handle this in the x86_exception_has_error_code() path to
avoid confusing readers with respect to exceptions that don't have an error code,
e.g.
else if (x86_exception_has_error_code(exc)) {
/* SVM doesn't provide the error code on #DF :-( */
if (exc == DF_VECTOR)
kvm_queue_exception_e(vcpu, exc, 0);
else
kvm_queue_exception_e(vcpu, exc, svm->vmcb->control.exit_info_1);
} else {
...
}
Alternatively, can we zero svm->vmcb->control.exit_info_1 on #DF to make it more
obvious that SVM leaves stale data in exit_info_1 (assuming that's true)? E.g.
...
if (exc == TS_VECTOR) {
...
} else if (x86_exception_has_error_code(exc)) {
/* SVM doesn't provide the error code on #DF :-( */
if (exc == DF_VECTOR)
svm->vmcb->control.exit_info_1 = 0;
kvm_queue_exception_e(vcpu, exc, svm->vmcb->control.exit_info_1);
} else {
...
}
> > +
> > + if (!(svm->force_intercept_exceptions_mask & (1 << exc)))
BIT(exc)
> > + return svm_handle_invalid_exit(vcpu, svm->vmcb->control.exit_code);
> > +
> > + if (exc == TS_VECTOR) {
> > + /*
> > + * SVM doesn't provide us with an error code to be able to
> > + * re-inject the #TS exception, so just disable its
> > + * intercept, and let the guest re-execute the instruction.
> > + */
> > + vmcb_clr_intercept(&svm->vmcb01.ptr->control,
> > + INTERCEPT_EXCEPTION_OFFSET + TS_VECTOR);
Maybe just disallow intercepting #TS altogether? Or does this fall into your
Win98 use case? :-)
> > + recalc_intercepts(svm);
> > + } else if (x86_exception_has_error_code(exc))
> > + kvm_queue_exception_e(vcpu, exc, err_code);
> > + else
> > + kvm_queue_exception(vcpu, exc);
> > + return 1;
> > +}
> > +
> > static bool is_erratum_383(void)
> > {
> > int err, i;
> > @@ -3065,6 +3131,10 @@ static int (*const svm_exit_handlers[])(struct kvm_vcpu *vcpu) = {
> > [SVM_EXIT_WRITE_DR5] = dr_interception,
> > [SVM_EXIT_WRITE_DR6] = dr_interception,
> > [SVM_EXIT_WRITE_DR7] = dr_interception,
> > +
> > + [SVM_EXIT_EXCP_BASE ...
> > + SVM_EXIT_EXCP_BASE + 31] = gen_exc_interception,
This generates a Sparse warning due to the duplicate initializer. IMO that's a
very good warning as I have zero idea how the compiler actually handles this
particular scenario, e.g. do later entries take priority, is it technically
"undefined" behavior, etc...
arch/x86/kvm/svm/svm.c:3065:10: warning: Initializer entry defined twice
arch/x86/kvm/svm/svm.c:3067:29: also defined here
I don't have a clever solution though :-(
> > +
> > [SVM_EXIT_EXCP_BASE + DB_VECTOR] = db_interception,
> > [SVM_EXIT_EXCP_BASE + BP_VECTOR] = bp_interception,
> > [SVM_EXIT_EXCP_BASE + UD_VECTOR] = ud_interception,
> > diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> > index 524d943f3efc..187ada7c5b03 100644
> > --- a/arch/x86/kvm/svm/svm.h
> > +++ b/arch/x86/kvm/svm/svm.h
> > @@ -196,6 +196,7 @@ struct vcpu_svm {
> > bool ghcb_sa_free;
> >
> > bool guest_state_loaded;
> > + u32 force_intercept_exceptions_mask;
> > };
> >
> > struct svm_cpu_data {
> > @@ -351,8 +352,11 @@ static inline void clr_exception_intercept(struct vcpu_svm *svm, u32 bit)
> > struct vmcb *vmcb = svm->vmcb01.ptr;
> >
> > WARN_ON_ONCE(bit >= 32);
> > - vmcb_clr_intercept(&vmcb->control, INTERCEPT_EXCEPTION_OFFSET + bit);
> >
> > + if ((1 << bit) & svm->force_intercept_exceptions_mask)
BIT(bit)
> > + return;
> > +
> > + vmcb_clr_intercept(&vmcb->control, INTERCEPT_EXCEPTION_OFFSET + bit);
> > recalc_intercepts(svm);
> > }
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