Re: [PATCH v2 7/7] KVM: VMX: Only context switch some PT MSRs when they exist

From: Xiaoyao Li
Date: Mon Oct 18 2021 - 10:09:16 EST

On 10/18/2021 9:08 PM, Paolo Bonzini wrote:
On 27/08/21 09:02, Xiaoyao Li wrote:
The enumeration of Intel PT feature doesn't guarantee the existence of
MSR_IA32_RTIT_OUTPUT_BASE, MSR_IA32_RTIT_OUTPUT_MASK and
MSR_IA32_RTIT_CR3_MATCH. They need to be detected from CPUID 0x14 PT
leaves.

Detect the existence of them in hardware_setup() and only context switch
them when they exist. Otherwise it will cause #GP when access them.

If intel_pt_validate_hw_cap is not fast enough, it should be optimized.
Something like this:

If I understand correctly, you mean we don't need to cache the existence with two variable, right?

diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
index 7f406c14715f..9c7167dcb719 100644
--- a/arch/x86/events/intel/pt.c
+++ b/arch/x86/events/intel/pt.c
@@ -41,13 +41,15 @@ static struct pt_pmu pt_pmu;
  * permitted values for certain bit fields).
  */
 #define PT_CAP(_n, _l, _r, _m)                        \
-    [PT_CAP_ ## _n] = { .name = __stringify(_n), .leaf = _l,    \
-                .reg = _r, .mask = _m }
+    [PT_CAP_ ## _n] = { .name = __stringify(_n),            \
+                .index = _l * PT_CPUID_REGS_NUM + _r,    \
+                .shift = __CONSTANT_FFS_32(_m),        \
+                .mask = _m }

 static struct pt_cap_desc {
     const char    *name;
-    u32        leaf;
-    u8        reg;
+    u16        index;
+    u8        shift;
     u32        mask;
 } pt_caps[] = {
     PT_CAP(max_subleaf,        0, CPUID_EAX, 0xffffffff),
@@ -71,10 +73,8 @@ static struct pt_cap_desc {
 u32 intel_pt_validate_cap(u32 *caps, enum pt_capabilities capability)
 {
     struct pt_cap_desc *cd = &pt_caps[capability];
-    u32 c = caps[cd->leaf * PT_CPUID_REGS_NUM + cd->reg];
-    unsigned int shift = __ffs(cd->mask);

-    return (c & cd->mask) >> shift;
+    return (caps[cd->index] & cd->mask) >> cd->shift;
 }
 EXPORT_SYMBOL_GPL(intel_pt_validate_cap);

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index 5e62e2383b7f..b4ee28d91b77 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -211,6 +211,17 @@ static inline int get_count_order_long(unsigned long l)
     return (int)fls_long(--l);
 }

+#define __CONSTANT_FFS_2(w) \
+    (((w) & 1) == 0)
+#define __CONSTANT_FFS_4(w) \
+    (((w) & 0x3) == 0 ? 2 + __CONSTANT_FFS_2((w) >> 2) : __CONSTANT_FFS_2((w)))
+#define __CONSTANT_FFS_8(w) \
+    (((w) & 0xf) == 0 ? 4 + __CONSTANT_FFS_4((w) >> 4) : __CONSTANT_FFS_4((w)))
+#define __CONSTANT_FFS_16(w) \
+    (((w) & 0xff) == 0 ? 8 + __CONSTANT_FFS_8((w) >> 8) : __CONSTANT_FFS_8((w)))
+#define __CONSTANT_FFS_32(w) \
+    (((w) & 0xffff) == 0 ? 16 + __CONSTANT_FFS_16((w) >> 16) : __CONSTANT_FFS_16((w)))
+
 /**
  * __ffs64 - find first set bit in a 64 bit word
  * @word: The 64 bit word


Paolo

Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
---
  arch/x86/kvm/vmx/vmx.c | 28 +++++++++++++++++++++-------
  1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 394ef4732838..6819fc470072 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -204,6 +204,9 @@ module_param(ple_window_max, uint, 0444);
  int __read_mostly pt_mode = PT_MODE_SYSTEM;
  module_param(pt_mode, int, S_IRUGO);
+static bool has_msr_rtit_cr3_match;
+static bool has_msr_rtit_output_x;
+
  static DEFINE_STATIC_KEY_FALSE(vmx_l1d_should_flush);
  static DEFINE_STATIC_KEY_FALSE(vmx_l1d_flush_cond);
  static DEFINE_MUTEX(vmx_l1d_flush_mutex);
@@ -1035,9 +1038,12 @@ static inline void pt_load_msr(struct pt_ctx *ctx, u32 addr_range)
      u32 i;
      wrmsrl(MSR_IA32_RTIT_STATUS, ctx->status);
-    wrmsrl(MSR_IA32_RTIT_OUTPUT_BASE, ctx->output_base);
-    wrmsrl(MSR_IA32_RTIT_OUTPUT_MASK, ctx->output_mask);
-    wrmsrl(MSR_IA32_RTIT_CR3_MATCH, ctx->cr3_match);
+    if (has_msr_rtit_output_x) {
+        wrmsrl(MSR_IA32_RTIT_OUTPUT_BASE, ctx->output_base);
+        wrmsrl(MSR_IA32_RTIT_OUTPUT_MASK, ctx->output_mask);
+    }
+    if (has_msr_rtit_cr3_match)
+        wrmsrl(MSR_IA32_RTIT_CR3_MATCH, ctx->cr3_match);
      for (i = 0; i < addr_range; i++) {
          wrmsrl(MSR_IA32_RTIT_ADDR0_A + i * 2, ctx->addr_a[i]);
          wrmsrl(MSR_IA32_RTIT_ADDR0_B + i * 2, ctx->addr_b[i]);
@@ -1049,9 +1055,12 @@ static inline void pt_save_msr(struct pt_ctx *ctx, u32 addr_range)
      u32 i;
      rdmsrl(MSR_IA32_RTIT_STATUS, ctx->status);
-    rdmsrl(MSR_IA32_RTIT_OUTPUT_BASE, ctx->output_base);
-    rdmsrl(MSR_IA32_RTIT_OUTPUT_MASK, ctx->output_mask);
-    rdmsrl(MSR_IA32_RTIT_CR3_MATCH, ctx->cr3_match);
+    if (has_msr_rtit_output_x) {
+        rdmsrl(MSR_IA32_RTIT_OUTPUT_BASE, ctx->output_base);
+        rdmsrl(MSR_IA32_RTIT_OUTPUT_MASK, ctx->output_mask);
+    }
+    if (has_msr_rtit_cr3_match)
+        rdmsrl(MSR_IA32_RTIT_CR3_MATCH, ctx->cr3_match);
      for (i = 0; i < addr_range; i++) {
          rdmsrl(MSR_IA32_RTIT_ADDR0_A + i * 2, ctx->addr_a[i]);
          rdmsrl(MSR_IA32_RTIT_ADDR0_B + i * 2, ctx->addr_b[i]);
@@ -7883,8 +7892,13 @@ static __init int hardware_setup(void)
      if (pt_mode != PT_MODE_SYSTEM && pt_mode != PT_MODE_HOST_GUEST)
          return -EINVAL;
-    if (!enable_ept || !cpu_has_vmx_intel_pt())
+    if (!enable_ept || !cpu_has_vmx_intel_pt()) {
          pt_mode = PT_MODE_SYSTEM;
+    } else if (boot_cpu_has(X86_FEATURE_INTEL_PT)) {
+        has_msr_rtit_cr3_match = intel_pt_validate_hw_cap(PT_CAP_cr3_filtering);
+        has_msr_rtit_output_x = intel_pt_validate_hw_cap(PT_CAP_topa_output) ||
+ intel_pt_validate_hw_cap(PT_CAP_single_range_output);
+    }
      setup_default_sgx_lepubkeyhash();