[Qemu-devel] [PATCH 1/3] KVM: Add new -cpu best

[Alexander Graf]

During discussions on whether to support -cpu host in SLE, I found myself
disagreeing to the thought, because it potentially opens a big can of worms
for potential bugs. But if I already am so opposed to it for SLE, how can
it possibly be reasonable to default to -cpu host in upstream QEMU? And what
would a sane default look like?

So I had this idea of looping through all available CPU definitions. We can
pretty well tell if our host is able to execute any of them by checking the
respective flags and seeing if our host has all features the CPU definition
requires. With that, we can create a -cpu type that would fall back to the
"best known CPU definition" that our host can fulfill. On my Phenom II
system for example, that would be -cpu phenom.

With this approach we can test and verify that CPU types actually work at
any random user setup, because we can always verify that all the -cpu types
we ship actually work. And we only default to some clever mechanism that
chooses from one of these.

Signed-off-by: Alexander Graf <address@hidden>

---

v2 -> v3:

  - clarify commit message
  - fix avi's style comments
---
 target-i386/cpu.c |   72 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 72 insertions(+), 0 deletions(-)

diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index 5521709..a5d9468 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -558,6 +558,76 @@ static int cpu_x86_fill_host(x86_def_t *x86_cpu_def)
     return 0;
 }
 
+/* Are all guest feature bits present on the host? */
+static bool cpu_x86_feature_subset(uint32_t host, uint32_t guest)
+{
+    return !(guest & ~host);
+}
+
+/* Does the host support all the features of the CPU definition? */
+static bool cpu_x86_fits_host(x86_def_t *x86_cpu_def)
+{
+    uint32_t eax = 0, ebx = 0, ecx = 0, edx = 0;
+
+    host_cpuid(0x0, 0, &eax, &ebx, &ecx, &edx);
+    if (x86_cpu_def->level > eax) {
+        return false;
+    }
+    if ((x86_cpu_def->vendor1 != ebx) ||
+        (x86_cpu_def->vendor2 != edx) ||
+        (x86_cpu_def->vendor3 != ecx)) {
+        return false;
+    }
+
+    host_cpuid(0x1, 0, &eax, &ebx, &ecx, &edx);
+    if (!cpu_x86_feature_subset(ecx, x86_cpu_def->ext_features) ||
+        !cpu_x86_feature_subset(edx, x86_cpu_def->features)) {
+        return false;
+    }
+
+    host_cpuid(0x80000000, 0, &eax, &ebx, &ecx, &edx);
+    if (x86_cpu_def->xlevel > eax) {
+        return false;
+    }
+
+    host_cpuid(0x80000001, 0, &eax, &ebx, &ecx, &edx);
+    if (!cpu_x86_feature_subset(edx, x86_cpu_def->ext2_features) ||
+        !cpu_x86_feature_subset(ecx, x86_cpu_def->ext3_features)) {
+        return false;
+    }
+
+    return true;
+}
+
+/* Returns true when new_def is higher versioned than old_def */
+static int cpu_x86_fits_higher(x86_def_t *new_def, x86_def_t *old_def)
+{
+    int old_fammod = (old_def->family << 24) | (old_def->model << 8)
+                   | (old_def->stepping);
+    int new_fammod = (new_def->family << 24) | (new_def->model << 8)
+                   | (new_def->stepping);
+
+    return new_fammod > old_fammod;
+}
+
+static void cpu_x86_fill_best(x86_def_t *x86_cpu_def)
+{
+    x86_def_t *def;
+
+    x86_cpu_def->family = 0;
+    x86_cpu_def->model = 0;
+    for (def = x86_defs; def; def = def->next) {
+        if (cpu_x86_fits_host(def) && cpu_x86_fits_higher(def, x86_cpu_def)) {
+            *x86_cpu_def = *def;
+        }
+    }
+
+    if (!x86_cpu_def->family && !x86_cpu_def->model) {
+        fprintf(stderr, "No fitting CPU model found!\n");
+        exit(1);
+    }
+}
+
 static int unavailable_host_feature(struct model_features_t *f, uint32_t mask)
 {
     int i;
@@ -878,6 +948,8 @@ static int cpu_x86_find_by_name(x86_def_t *x86_cpu_def, 
const char *cpu_model)
             break;
     if (kvm_enabled() && name && strcmp(name, "host") == 0) {
         cpu_x86_fill_host(x86_cpu_def);
+    } else if (kvm_enabled() && name && strcmp(name, "best") == 0) {
+        cpu_x86_fill_best(x86_cpu_def);
     } else if (!def) {
         goto error;
     } else {
-- 
1.6.0.2