[Qemu-devel] [PATCH v3 0/3] Add -cpu best support

[Qemu-devel] [PATCH v3 0/3] Add -cpu best support

[Alexander Graf]

[resend, forgot the v3, sorry]

This patch set implements support for a sane default CPU type, that plays the
middle ground between -cpu host (unmanagable test matrix) and -cpu qemu64 (breaks
assumptions wrt family/model numbers). It also makes it the default for -M pc,
so that users who don't specify a specific CPU type on the command line always
get a fast, yet reliable CPU type chosen for them.

For a detailed description, please see patch 1/3.

Alexander Graf (3):
  KVM: Add new -cpu best
  KVM: Use -cpu best as default on x86
  i386: KVM: List -cpu host and best in -cpu ?

 hw/pc_piix.c      |   34 +++++++++++++++++-----
 target-i386/cpu.c |   79 +++++++++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 102 insertions(+), 11 deletions(-)

[Qemu-devel] [PATCH v3 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 <agraf@suse.de>

---

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

[Qemu-devel] [PATCH v3 2/3] KVM: Use -cpu best as default on x86

[Alexander Graf]

When running QEMU without -cpu parameter, the user usually wants a sane
default. So far, we're using the qemu64/qemu32 CPU type, which basically
means "the maximum TCG can emulate".

That's a really good default when using TCG, but when running with KVM
we much rather want a default saying "the maximum performance I can get".

Fortunately we just added an option that gives us the best performance
while still staying safe on the testability side of things: -cpu best.
So all we need to do is make -cpu best the default when the user doesn't
explicitly specify a CPU type.

This fixes a lot of subtle breakage in the GNU toolchain (libgmp) which
hicks up on QEMU's non-existent CPU models.

This patch also adds a new pc-1.2 machine type to stay backwards compatible
with older versions of QEMU.

Signed-off-by: Alexander Graf <agraf@suse.de>

---

v1 -> v2:

  - rebase

v2 -> v3:

  - fix typo in commit message
---
 hw/pc_piix.c |   34 ++++++++++++++++++++++++++--------
 1 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/hw/pc_piix.c b/hw/pc_piix.c
index 0c0096f..a955bf8 100644
--- a/hw/pc_piix.c
+++ b/hw/pc_piix.c
@@ -127,7 +127,8 @@ static void pc_init1(MemoryRegion *system_memory,
                      const char *initrd_filename,
                      const char *cpu_model,
                      int pci_enabled,
-                     int kvmclock_enabled)
+                     int kvmclock_enabled,
+                     int may_cpu_best)
 {
     int i;
     ram_addr_t below_4g_mem_size, above_4g_mem_size;
@@ -149,6 +150,9 @@ static void pc_init1(MemoryRegion *system_memory,
     MemoryRegion *rom_memory;
     void *fw_cfg = NULL;
 
+    if (!cpu_model && kvm_enabled() && may_cpu_best) {
+        cpu_model = "best";
+    }
     pc_cpus_init(cpu_model);
 
     if (kvmclock_enabled) {
@@ -298,7 +302,21 @@ static void pc_init_pci(ram_addr_t ram_size,
              get_system_io(),
              ram_size, boot_device,
              kernel_filename, kernel_cmdline,
-             initrd_filename, cpu_model, 1, 1);
+             initrd_filename, cpu_model, 1, 1, 1);
+}
+
+static void pc_init_pci_oldcpu(ram_addr_t ram_size,
+                               const char *boot_device,
+                               const char *kernel_filename,
+                               const char *kernel_cmdline,
+                               const char *initrd_filename,
+                               const char *cpu_model)
+{
+    pc_init1(get_system_memory(),
+             get_system_io(),
+             ram_size, boot_device,
+             kernel_filename, kernel_cmdline,
+             initrd_filename, cpu_model, 1, 1, 0);
 }
 
 static void pc_init_pci_no_kvmclock(ram_addr_t ram_size,
@@ -312,7 +330,7 @@ static void pc_init_pci_no_kvmclock(ram_addr_t ram_size,
              get_system_io(),
              ram_size, boot_device,
              kernel_filename, kernel_cmdline,
-             initrd_filename, cpu_model, 1, 0);
+             initrd_filename, cpu_model, 1, 0, 0);
 }
 
 static void pc_init_isa(ram_addr_t ram_size,
@@ -328,7 +346,7 @@ static void pc_init_isa(ram_addr_t ram_size,
              get_system_io(),
              ram_size, boot_device,
              kernel_filename, kernel_cmdline,
-             initrd_filename, cpu_model, 0, 1);
+             initrd_filename, cpu_model, 0, 1, 0);
 }
 
 #ifdef CONFIG_XEN
@@ -380,7 +398,7 @@ static QEMUMachine pc_machine_v1_2 = {
 static QEMUMachine pc_machine_v1_1 = {
     .name = "pc-1.1",
     .desc = "Standard PC",
-    .init = pc_init_pci,
+    .init = pc_init_pci_oldcpu,
     .max_cpus = 255,
     .compat_props = (GlobalProperty[]) {
         PC_COMPAT_1_1,
@@ -415,7 +433,7 @@ static QEMUMachine pc_machine_v1_1 = {
 static QEMUMachine pc_machine_v1_0 = {
     .name = "pc-1.0",
     .desc = "Standard PC",
-    .init = pc_init_pci,
+    .init = pc_init_pci_oldcpu,
     .max_cpus = 255,
     .compat_props = (GlobalProperty[]) {
         PC_COMPAT_1_0,
@@ -430,7 +448,7 @@ static QEMUMachine pc_machine_v1_0 = {
 static QEMUMachine pc_machine_v0_15 = {
     .name = "pc-0.15",
     .desc = "Standard PC",
-    .init = pc_init_pci,
+    .init = pc_init_pci_oldcpu,
     .max_cpus = 255,
     .compat_props = (GlobalProperty[]) {
         PC_COMPAT_0_15,
@@ -462,7 +480,7 @@ static QEMUMachine pc_machine_v0_15 = {
 static QEMUMachine pc_machine_v0_14 = {
     .name = "pc-0.14",
     .desc = "Standard PC",
-    .init = pc_init_pci,
+    .init = pc_init_pci_oldcpu,
     .max_cpus = 255,
     .compat_props = (GlobalProperty[]) {
         PC_COMPAT_0_14, 
-- 
1.6.0.2

[Qemu-devel] [PATCH v3 3/3] i386: KVM: List -cpu host and best in -cpu ?

[Alexander Graf]

The kvm_enabled() helper doesn't work in a function as early as -cpu ?
yet. It also doesn't make sense to list the -cpu ? output conditional on
the -enable-kvm parameter. So let's always mention -cpu host in the
CPU list when KVM is supported on that configuration.

In addition, this patch also adds listing of -cpu best in the -cpu ?
list, so that people know that this option exists.

Signed-off-by: Alexander Graf <agraf@suse.de>
---
 target-i386/cpu.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index a5d9468..0000c77 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -1190,9 +1190,10 @@ void x86_cpu_list(FILE *f, fprintf_function cpu_fprintf, const char *optarg)
             (*cpu_fprintf)(f, "\n");
         }
     }
-    if (kvm_enabled()) {
-        (*cpu_fprintf)(f, "x86 %16s\n", "[host]");
-    }
+#ifdef CONFIG_KVM
+    (*cpu_fprintf)(f, "x86 %16s\n", "KVM only: [host]");
+    (*cpu_fprintf)(f, "x86 %16s\n", "KVM only: [best]");
+#endif
 }
 
 int cpu_x86_register(X86CPU *cpu, const char *cpu_model)
-- 
1.6.0.2