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Re: [Qemu-devel] [PATCH] target-i386: clear guest TSC on reset


From: Fernando Luis Vazquez Cao
Subject: Re: [Qemu-devel] [PATCH] target-i386: clear guest TSC on reset
Date: Thu, 05 Dec 2013 22:15:06 +0900
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.1.1

(2013/12/05 18:28), Paolo Bonzini wrote:
Il 05/12/2013 07:15, Fernando Luis Vázquez Cao ha scritto:
VCPU TSC is not cleared by a warm reset (*), which leaves many Linux
guests vulnerable to the overflow in cyc2ns_offset fixed by upstream
commit 9993bc635d01a6ee7f6b833b4ee65ce7c06350b1 ("sched/x86: Fix overflow
in cyc2ns_offset").

To put it in a nutshell, if a Linux guest without the patch above applied
has been up more than 208 days and attempts a warm reset chances are that
the newly booted kernel will panic or hang.

(*) Intel Xeon E5 processors show the same broken behavior due to
     the errata "TSC is Not Affected by Warm Reset" (Intel® Xeon®
     Processor E5 Family Specification Update - August 2013): "The
     TSC (Time Stamp Counter MSR 10H) should be cleared on
     reset. Due to this erratum the TSC is not affected by warm
     reset."

Cc: address@hidden
Cc: Will Auld <address@hidden>
Cc: Marcelo Tosatti <address@hidden>
Signed-off-by: Fernando Luis Vazquez Cao <address@hidden>
I agree that the bug is in QEMU.  One small nit in your patch is that
you should reset env->tsc_adjust and env->tsc in x86_cpu_reset.  This
would already be pretty good.

Yes, that is certainly cleaner (I should try not to take shortcuts...). I am attaching an updated patch (I apologize for not sending it inline - for reasons better left
untold I am writing this on a problematic email client :) ).



However, a bigger problem is that env->tsc is a useless duplicate of
"cpu_get_ticks() + env->tsc_adjust".  It would be nice to drop env->tsc
completely except for migration backwards compatibility.  Thus you can:

- fill in env->tsc as mentioned above from target-i386/machine.c's
cpu_pre_save function.  This guarantees backwards compatibility.

- add a function cpu_set_ticks(int64_t ticks) to cpus.c.  The function
does nothing if use_icount is true, otherwise it needs to have (roughly)
the opposite logic compared to cpu_get_ticks.  You then call this
function from x86_cpu_reset instead of setting env->tsc.  You can
similarly call this function from kvm_get_msrs.

- add a function kvm_set_ticks(int64_t ticks) to kvm-all.c and
kvm-stub.c.  For kvm-all.c it calls kvm_arch_set_ticks(CPUState *cpu,
int64_t ticks) in target-*/kvm.c.  The kvm_arch_set_tsc() function has a
dummy implementation for all architectures except x86.  For x86 it calls
KVM_SET_MSRS passing "ticks + env->tsc_offset".

- call kvm_set_ticks() from cpu_set_ticks() and cpu_enable_ticks()

Can you do this?

Can you pick my original fix first? I can do what you suggest in a follow-up
patch.

Thanks,
Fernando

Attachment: target-i386-clear-guest-TSC-on-reset-v2.patch
Description: Text document


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