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    SubjectRe: [RFC PATCH v5 067/104] KVM: x86: Allow to update cached values in kvm_user_return_msrs w/o wrmsr
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    On 3/4/22 20:49, 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>
    > ---
    > arch/x86/include/asm/kvm_host.h | 1 +
    > arch/x86/kvm/x86.c | 25 ++++++++++++++++++++-----
    > 2 files changed, 21 insertions(+), 5 deletions(-)
    >
    > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
    > index 8406f8b5ab74..b6396d11139e 100644
    > --- a/arch/x86/include/asm/kvm_host.h
    > +++ b/arch/x86/include/asm/kvm_host.h
    > @@ -1894,6 +1894,7 @@ int kvm_pv_send_ipi(struct kvm *kvm, unsigned long ipi_bitmap_low,
    > int kvm_add_user_return_msr(u32 msr);
    > int kvm_find_user_return_msr(u32 msr);
    > 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 66400810d54f..45e8a02e99bf 100644
    > --- a/arch/x86/kvm/x86.c
    > +++ b/arch/x86/kvm/x86.c
    > @@ -427,6 +427,15 @@ static void kvm_user_return_msr_cpu_online(void)
    > }
    > }
    >
    > +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)
    > {
    > unsigned int cpu = smp_processor_id();
    > @@ -441,15 +450,21 @@ 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);
    > +
    > + msrs->values[slot].curr = value;
    > + kvm_user_return_register_notifier(msrs);
    > +}
    > +EXPORT_SYMBOL_GPL(kvm_user_return_update_cache);
    > +
    > static void drop_user_return_notifiers(void)
    > {
    > unsigned int cpu = smp_processor_id();

    Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

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