Re: [PATCH] Enabling IA32_TSC_ADJUST for guest VM

[Avi Kivity <avi@redhat.com>]

On 10/09/2012 04:27 PM, Marcelo Tosatti wrote:
> On Tue, Oct 09, 2012 at 04:26:32PM +0200, Avi Kivity wrote:
>> On 10/09/2012 04:24 PM, Marcelo Tosatti wrote:
>> > On Tue, Oct 09, 2012 at 02:12:18PM +0200, Avi Kivity wrote:
>> >> On 10/08/2012 07:30 PM, Marcelo Tosatti wrote:
>> >> > 
>> >> > From Intel's manual:
>> >> > 
>> >> > • If an execution of WRMSR to the IA32_TIME_STAMP_COUNTER MSR adds (or
>> >> > subtracts) value X from the TSC,
>> >> > the logical processor also adds (or subtracts) value X from the
>> >> > IA32_TSC_ADJUST MSR.
>> >> > 
>> >> > This is not handled in the patch. 
>> >> > 
>> >> > To support migration, it will be necessary to differentiate between
>> >> > guest initiated and userspace-model initiated msr write. That is, 
>> >> > only guest initiated TSC writes should affect the value of 
>> >> > IA32_TSC_ADJUST MSR.
>> >> > 
>> >> > Avi, any better idea?
>> >> > 
>> >> 
>> >> I think we need that anyway, since there are some read-only MSRs that
>> >> need to be configured by the host (nvmx capabilities).  So if we add
>> >> that feature it will be useful elsewhere.  I don't think it's possible
>> >> to do it in any other way:
>> >> 
>> >> "Local offset value of the IA32_TSC for a
>> >> logical processor. Reset value is Zero. A
>> >> write to IA32_TSC will modify the local
>> >> offset in IA32_TSC_ADJUST and the
>> >> content of IA32_TSC, but does not affect
>> >> the internal invariant TSC hardware."
>> >> 
>> >> What we want to do is affect the internal invariant TSC hardware, so we
>> >> can't do that through the normal means.
>> >> 
>> >> btw, will tsc writes from userspace (after live migration) cause tsc
>> >> skew?  If so we should think how to model a guest-wide tsc.
>> > 
>> > No because there is an easy shortcut:
>> > 
>> >     if (level == KVM_PUT_FULL_STATE) {
>> >         /*
>> >          * KVM is yet unable to synchronize TSC values of multiple VCPUs
>> >          * on
>> >          * writeback. Until this is fixed, we only write the offset to
>> >          * SMP
>> >          * guests after migration, desynchronizing the VCPUs, but
>> >          * avoiding
>> >          * huge jump-backs that would occur without any writeback at
>> >          * all.
>> >          */
>> >         if (smp_cpus == 1 || env->tsc != 0) {
>> >             kvm_msr_entry_set(&msrs[n++], MSR_IA32_TSC, env->tsc);
>> >         }
>> >     }
>> 
>> Still we write back after migration.  So this needs to be fixed (or I
>> misunderstood you).
> 
> Handled by kvm_write_tsc() in x86.c. Is this what you mean?
> 

It will generate a call to ->write_tsc_offset().  Will the values be the
same for all vcpus?  Note the inputs won't be the same.


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error compiling committee.c: too many arguments to function