[RFC PATCH v3 53/59] KVM: x86: Add a helper function to restore 4 host MSRs on exit to user space

isaku.yamahata
2021-11-25T00:23:49Z

From: Chao Gao <chao.gao@intel.com>

The TDX module unconditionally reset 4 host MSRs (MSR_SYSCALL_MASK,
MSR_START, MSR_LSTAR, MSR_TSC_AUX) to architectural INIT state on exit from
TDX VM to KVM. KVM needs to save their values before TD enter and restore
them on exit to userspace.

Reuse current kvm_user_return mechanism and introduce a function to update
cached values and register the user return notifier in this new function.

The later patch will use the helper function to save/restore 4 host MSRs.

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 a2027bc14e41..17d6e4bcf84b 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1910,6 +1910,7 @@ 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 4dd8ec2641a2..09da7cbedb5f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -418,6 +418,15 @@ 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();
@@ -432,15 +441,21 @@ 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();
--
2.25.1