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Re: [PATCH 2/3] virhostcpu.c: skip non x86 hosts in virHostCPUGetMicrocodeVersion()



On a Monday in 2020, Daniel Henrique Barboza wrote:
Non-x86 archs does not have a 'microcode' version like x86. This is
covered already inside the function - just return 0 if no microcode
is found. Regardless of that, a read of /proc/cpuinfo is always made.
Each read will invoke the kernel to fill in the CPU details every time.

Now let's consider a non-x86 host, like a Power 9 server with 128 CPUs.
Each /proc/cpuinfo read will need to fetch data for each CPU and it
won't even matter because we know beforehand that PowerPC chips don't
have microcode information.

We can do better for non-x86 hosts by skipping this process entirely.

Signed-off-by: Daniel Henrique Barboza <danielhb413 gmail com>
---
src/cpu/cpu_x86.c            | 2 +-
src/qemu/qemu_capabilities.c | 4 ++--
src/util/virhostcpu.c        | 5 ++++-
src/util/virhostcpu.h        | 2 +-
4 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/src/cpu/cpu_x86.c b/src/cpu/cpu_x86.c
index 1e5cd93abb..aa982589be 100644
--- a/src/cpu/cpu_x86.c
+++ b/src/cpu/cpu_x86.c
@@ -2759,7 +2759,7 @@ virCPUx86GetHost(virCPUDefPtr cpu,
    }

    ret = x86DecodeCPUData(cpu, cpuData, models);
-    cpu->microcodeVersion = virHostCPUGetMicrocodeVersion();
+    cpu->microcodeVersion = virHostCPUGetMicrocodeVersion(cpuData->arch);

    /* Probing for TSC frequency makes sense only if the CPU supports
     * invariant TSC (Linux calls this constant_tsc in /proc/cpuinfo). */
diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
index ff6ba8c9e9..da743ecbe0 100644
--- a/src/qemu/qemu_capabilities.c
+++ b/src/qemu/qemu_capabilities.c
@@ -5502,7 +5502,7 @@ virQEMUCapsNewData(const char *binary,
                                           priv->runUid,
                                           priv->runGid,
                                           priv->hostCPUSignature,
-                                           virHostCPUGetMicrocodeVersion(),
+                                           virHostCPUGetMicrocodeVersion(priv->hostArch),
                                           priv->kernelVersion);
}

@@ -5636,7 +5636,7 @@ virQEMUCapsCacheLookup(virFileCachePtr cache,
    virQEMUCapsCachePrivPtr priv = virFileCacheGetPriv(cache);
    virQEMUCapsPtr ret = NULL;

-    priv->microcodeVersion = virHostCPUGetMicrocodeVersion();
+    priv->microcodeVersion = virHostCPUGetMicrocodeVersion(priv->hostArch);

    ret = virFileCacheLookup(cache, binary);

diff --git a/src/util/virhostcpu.c b/src/util/virhostcpu.c
index d7aa39c131..aafc3a14c6 100644
--- a/src/util/virhostcpu.c
+++ b/src/util/virhostcpu.c
@@ -1239,12 +1239,15 @@ virHostCPUGetKVMMaxVCPUs(void)
 * some reason.
 */
unsigned int
-virHostCPUGetMicrocodeVersion(void)
+virHostCPUGetMicrocodeVersion(virArch hostArch)

The corresponding non-Linux stub also needed the argument change.

I fixed that before pushing.

Jano

{
    g_autofree char *outbuf = NULL;
    char *cur;
    unsigned int version = 0;

+    if (!ARCH_IS_X86(hostArch))
+        return 0;
+
    if (virFileReadHeaderQuiet(CPUINFO_PATH, 4096, &outbuf) < 0) {
        VIR_DEBUG("Failed to read microcode version from %s: %s",
                  CPUINFO_PATH, g_strerror(errno));

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