]> git.baikalelectronics.ru Git - kernel.git/commit
KVM: x86: fix TSC matching
authorTomasz Grabiec <tgrabiec@cloudius-systems.com>
Tue, 24 Jun 2014 07:42:43 +0000 (09:42 +0200)
committerPaolo Bonzini <pbonzini@redhat.com>
Wed, 9 Jul 2014 16:09:57 +0000 (18:09 +0200)
commite3bb0999ac7e8dad7e71b4cef10b11c6eb98747a
treebf4d58245a974f0cbd2980e00cfd61cbb65f5474
parent628ea4bfb858fc180326d5590a98704e537ef15c
KVM: x86: fix TSC matching

I've observed kvmclock being marked as unstable on a modern
single-socket system with a stable TSC and qemu-1.6.2 or qemu-2.0.0.

The culprit was failure in TSC matching because of overflow of
kvm_arch::nr_vcpus_matched_tsc in case there were multiple TSC writes
in a single synchronization cycle.

Turns out that qemu does multiple TSC writes during init, below is the
evidence of that (qemu-2.0.0):

The first one:

 0xffffffffa08ff2b4 : vmx_write_tsc_offset+0xa4/0xb0 [kvm_intel]
 0xffffffffa04c9c05 : kvm_write_tsc+0x1a5/0x360 [kvm]
 0xffffffffa04cfd6b : kvm_arch_vcpu_postcreate+0x4b/0x80 [kvm]
 0xffffffffa04b8188 : kvm_vm_ioctl+0x418/0x750 [kvm]

The second one:

 0xffffffffa08ff2b4 : vmx_write_tsc_offset+0xa4/0xb0 [kvm_intel]
 0xffffffffa04c9c05 : kvm_write_tsc+0x1a5/0x360 [kvm]
 0xffffffffa090610d : vmx_set_msr+0x29d/0x350 [kvm_intel]
 0xffffffffa04be83b : do_set_msr+0x3b/0x60 [kvm]
 0xffffffffa04c10a8 : msr_io+0xc8/0x160 [kvm]
 0xffffffffa04caeb6 : kvm_arch_vcpu_ioctl+0xc86/0x1060 [kvm]
 0xffffffffa04b6797 : kvm_vcpu_ioctl+0xc7/0x5a0 [kvm]

 #0  kvm_vcpu_ioctl at /build/buildd/qemu-2.0.0+dfsg/kvm-all.c:1780
 #1  kvm_put_msrs at /build/buildd/qemu-2.0.0+dfsg/target-i386/kvm.c:1270
 #2  kvm_arch_put_registers at /build/buildd/qemu-2.0.0+dfsg/target-i386/kvm.c:1909
 #3  kvm_cpu_synchronize_post_init at /build/buildd/qemu-2.0.0+dfsg/kvm-all.c:1641
 #4  cpu_synchronize_post_init at /build/buildd/qemu-2.0.0+dfsg/include/sysemu/kvm.h:330
 #5  cpu_synchronize_all_post_init () at /build/buildd/qemu-2.0.0+dfsg/cpus.c:521
 #6  main at /build/buildd/qemu-2.0.0+dfsg/vl.c:4390

The third one:

 0xffffffffa08ff2b4 : vmx_write_tsc_offset+0xa4/0xb0 [kvm_intel]
 0xffffffffa04c9c05 : kvm_write_tsc+0x1a5/0x360 [kvm]
 0xffffffffa090610d : vmx_set_msr+0x29d/0x350 [kvm_intel]
 0xffffffffa04be83b : do_set_msr+0x3b/0x60 [kvm]
 0xffffffffa04c10a8 : msr_io+0xc8/0x160 [kvm]
 0xffffffffa04caeb6 : kvm_arch_vcpu_ioctl+0xc86/0x1060 [kvm]
 0xffffffffa04b6797 : kvm_vcpu_ioctl+0xc7/0x5a0 [kvm]

 #0  kvm_vcpu_ioctl at /build/buildd/qemu-2.0.0+dfsg/kvm-all.c:1780
 #1  kvm_put_msrs  at /build/buildd/qemu-2.0.0+dfsg/target-i386/kvm.c:1270
 #2  kvm_arch_put_registers  at /build/buildd/qemu-2.0.0+dfsg/target-i386/kvm.c:1909
 #3  kvm_cpu_synchronize_post_reset  at /build/buildd/qemu-2.0.0+dfsg/kvm-all.c:1635
 #4  cpu_synchronize_post_reset  at /build/buildd/qemu-2.0.0+dfsg/include/sysemu/kvm.h:323
 #5  cpu_synchronize_all_post_reset () at /build/buildd/qemu-2.0.0+dfsg/cpus.c:512
 #6  main  at /build/buildd/qemu-2.0.0+dfsg/vl.c:4482

The fix is to count each vCPU only once when matched, so that
nr_vcpus_matched_tsc holds the size of the matched set. This is
achieved by reusing generation counters. Every vCPU with
this_tsc_generation == cur_tsc_generation is in the matched set. The
match set is cleared by setting cur_tsc_generation to a value which no
other vCPU is set to (by incrementing it).

I needed to bump up the counter size form u8 to u64 to ensure it never
overflows. Otherwise in cases TSC is not written the same number of
times on each vCPU the counter could overflow and incorrectly indicate
some vCPUs as being in the matched set. This scenario seems unlikely
but I'm not sure if it can be disregarded.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
arch/x86/include/asm/kvm_host.h
arch/x86/kvm/x86.c