Messages in this thread | | | Date | Mon, 14 Nov 2022 15:36:27 +0800 | Subject | Re: [PATCH v10 071/108] KVM: x86: Allow to update cached values in kvm_user_return_msrs w/o wrmsr | From | Binbin Wu <> |
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On 10/30/2022 2:23 PM, isaku.yamahata@intel.com wrote: > From: Chao Gao <chao.gao@intel.com> > > Several MSRs are constant and only used in userspace(ring 3). But VMs may > have different values. KVM uses kvm_set_user_return_msr() to switch to > guest's values and leverages user return notifier to restore them when the > kernel is to return to userspace. To eliminate unnecessary wrmsr, KVM also > caches the value it wrote to an MSR last time. > > TDX module unconditionally resets some of these MSRs to architectural INIT > state on TD exit. It makes the cached values in kvm_user_return_msrs are > inconsistent with values in hardware. This inconsistency needs to be > fixed. Otherwise, it may mislead kvm_on_user_return() to skip restoring > some MSRs to the host's values. kvm_set_user_return_msr() can help correct > this case, but it is not optimal as it always does a wrmsr. So, introduce > a variation of kvm_set_user_return_msr() to update cached values and skip > that wrmsr. > > Signed-off-by: Chao Gao <chao.gao@intel.com> > Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com> > Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> > --- > arch/x86/include/asm/kvm_host.h | 1 + > arch/x86/kvm/x86.c | 26 +++++++++++++++++++++----- > 2 files changed, 22 insertions(+), 5 deletions(-) > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > index b225cdfac4bc..fdb00d96e954 100644 > --- a/arch/x86/include/asm/kvm_host.h > +++ b/arch/x86/include/asm/kvm_host.h > @@ -2082,6 +2082,7 @@ int kvm_add_user_return_msr(u32 msr); > int kvm_find_user_return_msr(u32 msr); > void kvm_user_return_msr_init_cpu(void); > int kvm_set_user_return_msr(unsigned index, u64 val, u64 mask); > +void kvm_user_return_update_cache(unsigned int index, u64 val); > > static inline bool kvm_is_supported_user_return_msr(u32 msr) > { > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 65541bfebb37..4d4b71c4cdb1 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -440,6 +440,15 @@ void kvm_user_return_msr_init_cpu(void) > } > EXPORT_SYMBOL_GPL(kvm_user_return_msr_init_cpu); > > +static void kvm_user_return_register_notifier(struct kvm_user_return_msrs *msrs) > +{ > + if (!msrs->registered) { > + msrs->urn.on_user_return = kvm_on_user_return; > + user_return_notifier_register(&msrs->urn); > + msrs->registered = true; > + } > +} > + > int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask) > { > struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs); > @@ -455,15 +464,22 @@ int kvm_set_user_return_msr(unsigned slot, u64 value, u64 mask) > return 1; > > msrs->values[slot].curr = value; > - if (!msrs->registered) { > - msrs->urn.on_user_return = kvm_on_user_return; > - user_return_notifier_register(&msrs->urn); > - msrs->registered = true; > - } > + kvm_user_return_register_notifier(msrs); > return 0; > } > EXPORT_SYMBOL_GPL(kvm_set_user_return_msr); > > +/* Update the cache, "curr", and register the notifier */ > +void kvm_user_return_update_cache(unsigned int slot, u64 value) > +{ > + struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs); > + > + WARN_ON_ONCE(!msrs->initialized); > + msrs->values[slot].curr = value;
If the value is the same as the cached one, can it just skip notifier registration?
> + kvm_user_return_register_notifier(msrs); > +} > +EXPORT_SYMBOL_GPL(kvm_user_return_update_cache); > + > static void drop_user_return_notifiers(void) > { > struct kvm_user_return_msrs *msrs = this_cpu_ptr(user_return_msrs);
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