[PATCH 8/9] x86/hyperv: Use TDX GHCI to access some MSRs in a TDX VM with the paravisor

Dexuan Cui
2023-08-11T22:20:24Z

When the paravisor is present, a SNP VM must use GHCB to access some
special MSRs, including HV_X64_MSR_GUEST_OS_ID and some SynIC MSRs.

Similarly, when the paravisor is present, a TDX VM must use TDX GHCI
to access the same MSRs.

Implement hv_tdx_read_msr() and hv_tdx_write_msr(), and use the helper
functions hv_ivm_msr_read() and hv_ivm_msr_write() to access the MSRs
in a unified way for SNP/TDX VMs with the paravisor.

Signed-off-by: Dexuan Cui <decui@microsoft.com>
---
arch/x86/hyperv/hv_init.c | 8 ++--
arch/x86/hyperv/ivm.c | 72 +++++++++++++++++++++++++++++++--
arch/x86/include/asm/mshyperv.h | 8 ++--
arch/x86/kernel/cpu/mshyperv.c | 8 ++--
4 files changed, 80 insertions(+), 16 deletions(-)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index 9c12a199ea62c..abd0a8bd3f15e 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -505,8 +505,8 @@ void __init hyperv_init(void)
guest_id = hv_generate_guest_id(LINUX_VERSION_CODE);
wrmsrl(HV_X64_MSR_GUEST_OS_ID, guest_id);

- /* Hyper-V requires to write guest os id via ghcb in SNP IVM. */
- hv_ghcb_msr_write(HV_X64_MSR_GUEST_OS_ID, guest_id);
+ /* With the paravisor, the VM must also write the ID via GHCB/GHCI */
+ hv_ivm_msr_write(HV_X64_MSR_GUEST_OS_ID, guest_id);

/* A TDX VM with no paravisor only uses TDX GHCI rather than hv_hypercall_pg */
if (hv_isolation_type_tdx() && !hyperv_paravisor_present)
@@ -595,7 +595,7 @@ void __init hyperv_init(void)

clean_guest_os_id:
wrmsrl(HV_X64_MSR_GUEST_OS_ID, 0);
- hv_ghcb_msr_write(HV_X64_MSR_GUEST_OS_ID, 0);
+ hv_ivm_msr_write(HV_X64_MSR_GUEST_OS_ID, 0);
cpuhp_remove_state(cpuhp);
free_ghcb_page:
free_percpu(hv_ghcb_pg);
@@ -616,7 +616,7 @@ void hyperv_cleanup(void)

/* Reset our OS id */
wrmsrl(HV_X64_MSR_GUEST_OS_ID, 0);
- hv_ghcb_msr_write(HV_X64_MSR_GUEST_OS_ID, 0);
+ hv_ivm_msr_write(HV_X64_MSR_GUEST_OS_ID, 0);

/*
* Reset hypercall page reference before reset the page,
diff --git a/arch/x86/hyperv/ivm.c b/arch/x86/hyperv/ivm.c
index 0d54bc8b06b4a..b8fb1557c1986 100644
--- a/arch/x86/hyperv/ivm.c
+++ b/arch/x86/hyperv/ivm.c
@@ -184,7 +184,49 @@ bool hv_ghcb_negotiate_protocol(void)
return true;
}

-void hv_ghcb_msr_write(u64 msr, u64 value)
+#define EXIT_REASON_MSR_READ 31
+#define EXIT_REASON_MSR_WRITE 32
+
+static void hv_tdx_read_msr(u64 msr, u64 *val)
+{
+ struct tdx_hypercall_args args = {
+ .r10 = TDX_HYPERCALL_STANDARD,
+ .r11 = EXIT_REASON_MSR_READ,
+ .r12 = msr,
+ };
+
+#ifdef CONFIG_INTEL_TDX_GUEST
+ u64 ret = __tdx_hypercall_ret(&args);
+#else
+ u64 ret = HV_STATUS_INVALID_PARAMETER;
+#endif
+
+ if (WARN_ONCE(ret, "Failed to emulate MSR read: %lld\n", ret))
+ *val = 0;
+ else
+ *val = args.r11;
+}
+
+static void hv_tdx_write_msr(u64 msr, u64 val)
+{
+ struct tdx_hypercall_args args = {
+ .r10 = TDX_HYPERCALL_STANDARD,
+ .r11 = EXIT_REASON_MSR_WRITE,
+ .r12 = msr,
+ .r13 = val,
+ };
+
+#ifdef CONFIG_INTEL_TDX_GUEST
+ u64 ret = __tdx_hypercall(&args);
+#else
+ u64 ret = HV_STATUS_INVALID_PARAMETER;
+ (void)args;
+#endif
+
+ WARN_ONCE(ret, "Failed to emulate MSR write: %lld\n", ret);
+}
+
+static void hv_ghcb_msr_write(u64 msr, u64 value)
{
union hv_ghcb *hv_ghcb;
void **ghcb_base;
@@ -212,9 +254,20 @@ void hv_ghcb_msr_write(u64 msr, u64 value)

local_irq_restore(flags);
}
-EXPORT_SYMBOL_GPL(hv_ghcb_msr_write);

-void hv_ghcb_msr_read(u64 msr, u64 *value)
+void hv_ivm_msr_write(u64 msr, u64 value)
+{
+ if (!hyperv_paravisor_present)
+ return;
+
+ if (hv_isolation_type_tdx())
+ hv_tdx_write_msr(msr, value);
+ else if (hv_isolation_type_snp())
+ hv_ghcb_msr_write(msr, value);
+}
+EXPORT_SYMBOL_GPL(hv_ivm_msr_write);
+
+static void hv_ghcb_msr_read(u64 msr, u64 *value)
{
union hv_ghcb *hv_ghcb;
void **ghcb_base;
@@ -244,7 +297,18 @@ void hv_ghcb_msr_read(u64 msr, u64 *value)
| ((u64)lower_32_bits(hv_ghcb->ghcb.save.rdx) << 32);
local_irq_restore(flags);
}
-EXPORT_SYMBOL_GPL(hv_ghcb_msr_read);
+
+void hv_ivm_msr_read(u64 msr, u64 *value)
+{
+ if (!hyperv_paravisor_present)
+ return;
+
+ if (hv_isolation_type_tdx())
+ hv_tdx_read_msr(msr, value);
+ else if (hv_isolation_type_snp())
+ hv_ghcb_msr_read(msr, value);
+}
+EXPORT_SYMBOL_GPL(hv_ivm_msr_read);

/*
* hv_mark_gpa_visibility - Set pages visible to host via hvcall.
diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index 9fa31dce69727..3f22324ef2e25 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -255,15 +255,15 @@ int hv_map_ioapic_interrupt(int ioapic_id, bool level, int vcpu, int vector,
int hv_unmap_ioapic_interrupt(int ioapic_id, struct hv_interrupt_entry *entry);

#ifdef CONFIG_AMD_MEM_ENCRYPT
-void hv_ghcb_msr_write(u64 msr, u64 value);
-void hv_ghcb_msr_read(u64 msr, u64 *value);
+void hv_ivm_msr_write(u64 msr, u64 value);
+void hv_ivm_msr_read(u64 msr, u64 *value);
bool hv_ghcb_negotiate_protocol(void);
void __noreturn hv_ghcb_terminate(unsigned int set, unsigned int reason);
void hv_vtom_init(void);
int hv_snp_boot_ap(int cpu, unsigned long start_ip);
#else
-static inline void hv_ghcb_msr_write(u64 msr, u64 value) {}
-static inline void hv_ghcb_msr_read(u64 msr, u64 *value) {}
+static inline void hv_ivm_msr_write(u64 msr, u64 value) {}
+static inline void hv_ivm_msr_read(u64 msr, u64 *value) {}
static inline bool hv_ghcb_negotiate_protocol(void) { return false; }
static inline void hv_ghcb_terminate(unsigned int set, unsigned int reason) {}
static inline void hv_vtom_init(void) {}
diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index ddcc62185e4ae..fb585d3b080b1 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -72,8 +72,8 @@ u64 hv_get_non_nested_register(unsigned int reg)
{
u64 value;

- if (hv_is_synic_reg(reg) && hv_isolation_type_snp())
- hv_ghcb_msr_read(reg, &value);
+ if (hv_is_synic_reg(reg) && hyperv_paravisor_present)
+ hv_ivm_msr_read(reg, &value);
else
rdmsrl(reg, value);
return value;
@@ -82,8 +82,8 @@ EXPORT_SYMBOL_GPL(hv_get_non_nested_register);

void hv_set_non_nested_register(unsigned int reg, u64 value)
{
- if (hv_is_synic_reg(reg) && hv_isolation_type_snp()) {
- hv_ghcb_msr_write(reg, value);
+ if (hv_is_synic_reg(reg) && hyperv_paravisor_present) {
+ hv_ivm_msr_write(reg, value);

/* Write proxy bit via wrmsl instruction */
if (hv_is_sint_reg(reg))
--
2.25.1