On Tue, Oct 09, 2012 at 04:10:30PM +0000, Auld, Will wrote: > I am just testing the second version of this patch. It addresses all the comments so far except Marcelo's issue with breaking the function compute_guest_tsc(). Lets try to merge the missing patch from Zachary first (that'll make it clear). > > I needed to put the call for updating the TSC_ADJUST_MSR in kvm_write_tsc() to ensure it is only called from user space. Other changes added to vmcs offset should not be tracked in TSC_ADJUST_MSR. Please have a separate, earlier patch making that explicit (by passing a bool to kvm_x86_ops->set_msr then to kvm_set_msr_common). "that" = whether msr write is guest initiated or not. > I had some trouble with the order of initialization during live migration. TSC_ADJUST is initialized first but then wiped out by multiple initializations of tsc. The fix for this is to not update TSC_ADJUST if the vmcs offset is not actually changing with the tsc write. So, after migration outcome is that vmcs offset gets defined independent from the migrating value of TSC_ADJUST. I believe this is what we want to happen. Can you please be more explicit regarding "wiped out by multiple initializations of tsc" ? It is probably best to maintain TSC_ADJUST separately, in software, and then calculate TSC_OFFSET. > Thanks, > > Will > > -----Original Message----- > From: Avi Kivity [mailto:avi@xxxxxxxxxx] > Sent: Tuesday, October 09, 2012 5:12 AM > To: Marcelo Tosatti > Cc: Auld, Will; kvm@xxxxxxxxxxxxxxx; Zhang, Xiantao > Subject: Re: [PATCH] Enabling IA32_TSC_ADJUST for guest VM > > 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. > > -- > error compiling committee.c: too many arguments to function > N?????r??y????b?X??ǧv?^?){.n?+????h????ܨ}???Ơz?&j:+v???????zZ+??+zf???h???~????i???z? ?w?????????&?)ߢf -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
