Re: [Xen-devel] [PATCH v5 RESEND 15/17] x86/VPMU: NMI-based VPMU support

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 04/26/2014 04:48 AM, Tian, Kevin wrote:
> > From: Boris Ostrovsky [mailto:boris.ostrovsky@xxxxxxxxxx]
> > Sent: Wednesday, April 23, 2014 8:51 PM
> >
> > Add support for using NMIs as PMU interrupts.
> >
> > Most of processing is still performed by vpmu_do_interrupt(). However, since
> > certain operations are not NMI-safe we defer them to a softint that
> > vpmu_do_interrupt()
> > will schedule:
> > * For PV guests that would be send_guest_vcpu_virq()
> > * For HVM guests it's VLAPIC accesses and hvm_get_segment_register() (the
> > later
> > can be called in privileged profiling mode when the interrupted guest is an
> > HVM one).
> >
> > With send_guest_vcpu_virq() and hvm_get_segment_register() for PV(H) and
> > vlapic
> > accesses for HVM moved to sofint, the only routines/macros that
> > vpmu_do_interrupt()
> > calls in NMI mode are:
> > * memcpy()
> > * querying domain type (is_XX_domain())
> > * guest_cpu_user_regs()
> > * XLAT_cpu_user_regs()
> > * raise_softirq()
> > * vcpu_vpmu()
> > * vpmu_ops->arch_vpmu_save()
> > * vpmu_ops->do_interrupt() (in the future for PVH support)
> >
> > The latter two only access PMU MSRs with {rd,wr}msrl() (not the _safe
> > versions
> > which would not be NMI-safe).
> >
> > Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> if a SCHEDULE_SOFTIRQ happens before NMI_SOFTIRQ, will it cause any race
> condition when accessing previous recorded content in nmi softirq handler?
I think you are right.

I in fact had an earlier version where I'd put the record into a linked 
list but when I later looked at that code again I took it out because I 
thought it was unnecessary. But now that you mentioned it I may need to 
resurrect that code. Let me think about this again.
-boris


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