[PATCH v3 2/2] x86, cpuid: allow cpuid_read() to schedule

[Eric Dumazet]

I noticed high latencies caused by a daemon periodically reading various
MSR and cpuid on all cpus. KASAN kernels would see ~10ms latencies
simply reading one cpuid. Even without KASAN, sending IPI to CPU
in deep sleep state or blocking hard IRQ in a a long section,
then waiting for the answer can consume hundreds of usec or more.

Switching to smp_call_function_single_async() and a completion
allows to reschedule and not burn cpu cycles.

Signed-off-by: Eric Dumazet <eduma...@google.com>
Cc: "H. Peter Anvin" <h...@zytor.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Borislav Petkov <b...@alien8.de>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: Hugh Dickins <hu...@google.com>
---
 arch/x86/kernel/cpuid.c | 34 ++++++++++++++++++++++++++--------
 1 file changed, 26 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/cpuid.c b/arch/x86/kernel/cpuid.c
index 
0931a105ffe16cde4640e759efa600b23a756d84..1d300f96df4b316dbe3392c8221467cfd8593272
 100644
--- a/arch/x86/kernel/cpuid.c
+++ b/arch/x86/kernel/cpuid.c
@@ -40,6 +40,7 @@
 #include <linux/notifier.h>
 #include <linux/uaccess.h>
 #include <linux/gfp.h>
+#include <linux/completion.h>
 
 #include <asm/processor.h>
 #include <asm/msr.h>
@@ -47,19 +48,27 @@
 static struct class *cpuid_class;
 static enum cpuhp_state cpuhp_cpuid_state;
 
+struct cpuid_regs_done {
+       struct cpuid_regs regs;
+       struct completion done;
+};
+
 static void cpuid_smp_cpuid(void *cmd_block)
 {
-       struct cpuid_regs *cmd = (struct cpuid_regs *)cmd_block;
+       struct cpuid_regs_done *cmd = cmd_block;
+
+       cpuid_count(cmd->regs.eax, cmd->regs.ecx,
+                   &cmd->regs.eax, &cmd->regs.ebx,
+                   &cmd->regs.ecx, &cmd->regs.edx);
 
-       cpuid_count(cmd->eax, cmd->ecx,
-                   &cmd->eax, &cmd->ebx, &cmd->ecx, &cmd->edx);
+       complete(&cmd->done);
 }
 
 static ssize_t cpuid_read(struct file *file, char __user *buf,
                          size_t count, loff_t *ppos)
 {
        char __user *tmp = buf;
-       struct cpuid_regs cmd;
+       struct cpuid_regs_done cmd;
        int cpu = iminor(file_inode(file));
        u64 pos = *ppos;
        ssize_t bytes = 0;
@@ -68,19 +77,28 @@ static ssize_t cpuid_read(struct file *file, char __user 
*buf,
        if (count % 16)
                return -EINVAL; /* Invalid chunk size */
 
+       init_completion(&cmd.done);
        for (; count; count -= 16) {
-               cmd.eax = pos;
-               cmd.ecx = pos >> 32;
-               err = smp_call_function_single(cpu, cpuid_smp_cpuid, &cmd, 1);
+               call_single_data_t csd = {
+                       .func = cpuid_smp_cpuid,
+                       .info = &cmd,
+               };
+
+               cmd.regs.eax = pos;
+               cmd.regs.ecx = pos >> 32;
+
+               err = smp_call_function_single_async(cpu, &csd);
                if (err)
                        break;
-               if (copy_to_user(tmp, &cmd, 16)) {
+               wait_for_completion(&cmd.done);
+               if (copy_to_user(tmp, &cmd.regs, 16)) {
                        err = -EFAULT;
                        break;
                }
                tmp += 16;
                bytes += 16;
                *ppos = ++pos;
+               reinit_completion(&cmd.done);
        }
 
        return bytes ? bytes : err;
-- 
2.17.0.rc0.231.g781580f067-goog