Re: [PATCH] target/i386: Return right size value after dynamic xfeature enabled

[Paolo Bonzini]

On 3/24/22 04:18, Yang Zhong wrote:
The kvm_arch_get_supported_cpuid() only call KVM_GET_SUPPORTED_CPUID one
time, so the cpuid buffer information still keep older value. Once Qemu
enable new dynamic xfeature, like XTILEDATA, the cpuid[0D,0].{EBX,ECX}
still return older value.
This patch can return right size value in kvm_init_xsave() if XTILEDATA
has been enabled by arch_prctl.

assert(kvm_arch_get_supported_cpuid(kvm_state, 0xd, 0, R_ECX) <=
            env->xsave_buf_len);

Signed-off-by: Yang Zhong <yang.zh...@intel.com>

I don't understand, is this a bugfix for an assertion failure or just a 
cleanup?

Either way, while I like the idea of modifying 
kvm_arch_get_supported_cpuid, I think the right thing to do is to just 
use has_xsave2 as the return value if it is nonzero.  And then 
kvm_init_xsave can just do

if (!has_xsave) {
    return;
}
env->xsave_buf_len = kvm_arch_get_supported_cpuid(kvm_state, 0xd, 0, R_ECX);

without the assertion that is now obvious.

Paolo

---
  target/i386/cpu.h     |  3 +++
  target/i386/kvm/kvm.c | 15 +++++++++++++--
  2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index 5e406088a9..814ba4020b 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -565,6 +565,9 @@ typedef enum X86Seg {
  #define ESA_FEATURE_ALIGN64_MASK        (1U << ESA_FEATURE_ALIGN64_BIT)
  #define ESA_FEATURE_XFD_MASK            (1U << ESA_FEATURE_XFD_BIT)
  
+#define ARCH_GET_XCOMP_GUEST_PERM       0x1024
+#define ARCH_REQ_XCOMP_GUEST_PERM       0x1025
+
  
  /* CPUID feature words */
  typedef enum FeatureWord {
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 06901c2a43..312d4fccf8 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -46,6 +46,7 @@
  #include "hw/i386/intel_iommu.h"
  #include "hw/i386/x86-iommu.h"
  #include "hw/i386/e820_memory_layout.h"
+#include "target/i386/cpu.h"
  
  #include "hw/pci/pci.h"
  #include "hw/pci/msi.h"
@@ -437,6 +438,18 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, 
uint32_t function,
              return ret;
          }
          ret = (reg == R_EAX) ? bitmask : bitmask >> 32;
+    } else if (function == 0xd && index == 0 &&
+               (reg == R_EBX || reg == R_ECX)) {
+        /*
+         * The value returned by KVM_GET_SUPPORTED_CPUID does not include
+         * features that already be enabled with the arch_prctl system call.
+         */
+        int rc = syscall(SYS_arch_prctl, ARCH_GET_XCOMP_GUEST_PERM, &bitmask);
+        if (rc) {
+            warn_report("prctl(ARCH_GET_XCOMP_GUEST_PERM) error: %d", rc);
+        } else if (bitmask & XSTATE_XTILE_DATA_MASK) {
+            ret += sizeof(XSaveXTILEDATA);
+        }
      } else if (function == 0x80000001 && reg == R_ECX) {
          /*
           * It's safe to enable TOPOEXT even if it's not returned by
@@ -5214,8 +5227,6 @@ bool kvm_arch_cpu_check_are_resettable(void)
      return !sev_es_enabled();
  }
  
-#define ARCH_REQ_XCOMP_GUEST_PERM       0x1025
-
  void kvm_request_xsave_components(X86CPU *cpu, uint64_t mask)
  {
      KVMState *s = kvm_state;