[RFC PATCH] clocksource: hyper-v: Enable the tsc_page for a TDX VM in TD mode

[Dexuan Cui]

A TDX VM on Hyper-V may run in TD mode or Partitioned TD mode (L2). For
the former, the VM has not enabled the Hyper-V TSC page (which is defined
in drivers/clocksource/hyperv_timer.c: "... tsc_pg __bss_decrypted ...")
because, for such a VM, the hypervisor requires that the page should be
shared, but currently the __bss_decrypted is not working for such a VM
yet.

Hyper-V TSC page can work as a clocksource device similar to KVM pv
clock, and it's also used by the Hyper-V timer code to get the current
time: see hv_init_tsc_clocksource(), which sets the global function
pointer hv_read_reference_counter to read_hv_clock_tsc(); when
Hyper-V TSC page is not enabled, hv_read_reference_counter defaults to
be drivers/hv/hv_common.c: __hv_read_ref_counter(), which is suboptimal
as it uses the slow MSR interface to get the time info.

The attribute __bss_decrypted was added for a native SNP VM by the
commit 45f46b1ac95e ("clocksource: hyper-v: Mark hyperv tsc page unencrypted in sev-snp enlightened guest")

The attribute works for SNP because of the commit below
commit b3f0907c71e0 ("x86/mm: Add .bss..decrypted section to hold shared variables")

The attribute is not working for TDX because __startup_64() ->
sme_postprocess_startup() is not for TDX; we can't just call
set_memory_decrypted() in sme_postprocess_startup() because
sme_postprocess_startup() runs too early and set_memory_decrypted() is not
working there yet.

This RFC patch calls set_memory_decrypted() in a later place, i.e., in
start_kernel() -> setup_arch() -> init_hypervisor_platform() ->
ms_hyperv_init_platform(), so set_memory_decrypted() works there;
accordingly, mem_encrypt_free_decrypted_mem() -> set_memory_encrypted()
must be called for TDX now.

When a TDX VM runs in Partitioned TD mode (L2), the Hyper-V TSC page should
be a private page, so set_memory_decrypted() should not be called for the
page in such a VM. Introduce a global variable "tdx_partitioned_td_l2" to
handle this type of VM differently.

BTW, the 4KB Hyper-V TSC page is enabled very early in
hv_init_tsc_clocksource(), where set_memory_decrypted() is not working yet,
so we can't simply call set_memory_decrypted() in hv_init_tsc_clocksource()
for a TDX VM in TD mode, and we need to get the attribute __bss_decrypted
to work for such a VM.

The changes to arch/x86/kernel/cpu/mshyperv.c and
arch/x86/mm/mem_encrypt_amd.c are not satisfying to me. I wish there
could be a better way to support __bss_decrypted for a native TDX VM
so that a TDX VM on KVM could also benefit from __bss_decrypted, if
some one wants to use it in future. BTW, kvm_init_platform() has
similar code for SNP.

This is just a RFC patch. I apprecite your insight. Thanks!

Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
---
 arch/x86/coco/core.c           | 15 +++++++++++++++
 arch/x86/coco/tdx/tdx.c        |  2 ++
 arch/x86/hyperv/ivm.c          |  3 ++-
 arch/x86/include/asm/tdx.h     |  1 +
 arch/x86/kernel/cpu/mshyperv.c |  8 ++++++--
 arch/x86/mm/mem_encrypt_amd.c  |  3 ++-
 6 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/arch/x86/coco/core.c b/arch/x86/coco/core.c
index b31ef2424d194..61cec405f1084 100644
--- a/arch/x86/coco/core.c
+++ b/arch/x86/coco/core.c
@@ -15,6 +15,7 @@
 
 #include <asm/archrandom.h>
 #include <asm/coco.h>
+#include <asm/tdx.h>
 #include <asm/processor.h>
 
 enum cc_vendor cc_vendor __ro_after_init = CC_VENDOR_NONE;
@@ -25,8 +26,22 @@ static struct cc_attr_flags {
 	      __resv		: 63;
 } cc_flags;
 
+static bool noinstr intel_cc_platform_td_l2(enum cc_attr attr)
+{
+	switch (attr) {
+	case CC_ATTR_GUEST_MEM_ENCRYPT:
+	case CC_ATTR_MEM_ENCRYPT:
+		return true;
+	default:
+		return false;
+	}
+}
+
 static bool noinstr intel_cc_platform_has(enum cc_attr attr)
 {
+	if (tdx_partitioned_td_l2)
+		return intel_cc_platform_td_l2(attr);
+
 	switch (attr) {
 	case CC_ATTR_GUEST_UNROLL_STRING_IO:
 	case CC_ATTR_HOTPLUG_DISABLED:
diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index abf3cd591afd3..8e6ab42add7c0 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -39,6 +39,8 @@
 
 #define TDREPORT_SUBTYPE_0	0
 
+bool tdx_partitioned_td_l2 __ro_after_init;
+
 /* Called from __tdx_hypercall() for unrecoverable failure */
 noinstr void __noreturn __tdx_hypercall_failed(void)
 {
diff --git a/arch/x86/hyperv/ivm.c b/arch/x86/hyperv/ivm.c
index 768d73de0d098..52cd44e507846 100644
--- a/arch/x86/hyperv/ivm.c
+++ b/arch/x86/hyperv/ivm.c
@@ -626,7 +626,7 @@ static bool hv_is_private_mmio(u64 addr)
 	return false;
 }
 
-void __init hv_vtom_init(void)
+void __init hv_vtom_init(void) /* TODO: rename the function for TDX */
 {
 	enum hv_isolation_type type = hv_get_isolation_type();
 
@@ -650,6 +650,7 @@ void __init hv_vtom_init(void)
 
 	case HV_ISOLATION_TYPE_TDX:
 		cc_vendor = CC_VENDOR_INTEL;
+		tdx_partitioned_td_l2 = true;
 		break;
 
 	default:
diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index eba178996d845..ddcc9ef82dc99 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -66,6 +66,7 @@ int tdx_mcall_get_report0(u8 *reportdata, u8 *tdreport);
 
 u64 tdx_hcall_get_quote(u8 *buf, size_t size);
 
+extern bool tdx_partitioned_td_l2;
 #else
 
 static inline void tdx_early_init(void) { };
diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index e0fd57a8ba840..7c336bc020c9f 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -18,6 +18,7 @@
 #include <linux/kexec.h>
 #include <linux/i8253.h>
 #include <linux/random.h>
+#include <linux/set_memory.h>
 #include <asm/processor.h>
 #include <asm/hypervisor.h>
 #include <asm/hyperv-tlfs.h>
@@ -449,8 +450,11 @@ static void __init ms_hyperv_init_platform(void)
 			ms_hyperv.hints &= ~HV_X64_APIC_ACCESS_RECOMMENDED;
 
 			if (!ms_hyperv.paravisor_present) {
-				/* To be supported: more work is required.  */
-				ms_hyperv.features &= ~HV_MSR_REFERENCE_TSC_AVAILABLE;
+				unsigned long vaddr = (unsigned long)__start_bss_decrypted;
+				unsigned long vaddr_end = (unsigned long)__end_bss_decrypted;
+
+				for (; vaddr < vaddr_end; vaddr += PMD_SIZE)
+					set_memory_decrypted(vaddr, PMD_SIZE >> PAGE_SHIFT);
 
 				/* HV_MSR_CRASH_CTL is unsupported. */
 				ms_hyperv.misc_features &= ~HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE;
diff --git a/arch/x86/mm/mem_encrypt_amd.c b/arch/x86/mm/mem_encrypt_amd.c
index 422602f6039b8..0ddb9e5d222c3 100644
--- a/arch/x86/mm/mem_encrypt_amd.c
+++ b/arch/x86/mm/mem_encrypt_amd.c
@@ -25,6 +25,7 @@
 #include <asm/fixmap.h>
 #include <asm/setup.h>
 #include <asm/mem_encrypt.h>
+#include <asm/tdx.h>
 #include <asm/bootparam.h>
 #include <asm/set_memory.h>
 #include <asm/cacheflush.h>
@@ -529,7 +530,7 @@ void __init mem_encrypt_free_decrypted_mem(void)
 	 * CC_ATTR_MEM_ENCRYPT, aren't necessarily equivalent in a Hyper-V VM
 	 * using vTOM, where sme_me_mask is always zero.
 	 */
-	if (sme_me_mask) {
+	if (sme_me_mask || (cc_vendor == CC_VENDOR_INTEL && !tdx_partitioned_td_l2)) {
 		r = set_memory_encrypted(vaddr, npages);
 		if (r) {
 			pr_warn("failed to free unused decrypted pages\n");
-- 
2.25.1