> -----Original Message-----
> From: Michael Kelley (EOSG)
> Sent: Tuesday, June 19, 2018 10:57 AM
> To: Vitaly Kuznetsov <vkuzn...@redhat.com>; x...@kernel.org
> Cc: de...@linuxdriverproject.org; linux-kernel@vger.kernel.org; KY
> Srinivasan <k...@microsoft.com>; Haiyang Zhang
> <haiya...@microsoft.com>; Stephen Hemminger
> <sthem...@microsoft.com>; Thomas Gleixner <t...@linutronix.de>; Ingo
> Molnar <mi...@redhat.com>; H. Peter Anvin <h...@zytor.com>; Tianyu Lan
> <tianyu....@microsoft.com>
> Subject: RE: [PATCH] x86/hyper-v: use cheaper
> HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE} hypercalls when possible
>
> > -----Original Message-----
> > From: linux-kernel-ow...@vger.kernel.org <linux-kernel-
> ow...@vger.kernel.org> On Behalf
> > Of Vitaly Kuznetsov
> > Sent: Friday, June 15, 2018 9:30 AM
> > To: x...@kernel.org
> > Cc: de...@linuxdriverproject.org; linux-kernel@vger.kernel.org; KY
> Srinivasan
> > <k...@microsoft.com>; Haiyang Zhang <haiya...@microsoft.com>;
> Stephen Hemminger
> > <sthem...@microsoft.com>; Thomas Gleixner <t...@linutronix.de>; Ingo
> Molnar
> > <mi...@redhat.com>; H. Peter Anvin <h...@zytor.com>; Tianyu Lan
> > <tianyu....@microsoft.com>
> > Subject: [PATCH] x86/hyper-v: use cheaper
> HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE}
> > hypercalls when possible
> >
> > While working on Hyper-V style PV TLB flush support in KVM I noticed that
> > real Windows guests use TLB flush hypercall in a somewhat smarter way:
> when
> > the flush needs to be performed on a subset of first 64 vCPUs or on all
> > present vCPUs Windows avoids more expensive hypercalls which support
> > sparse CPU sets and uses their 'cheap' counterparts. This means that
> > HV_X64_EX_PROCESSOR_MASKS_RECOMMENDED name is actually a
> misnomer: EX
> > hypercalls (which support sparse CPU sets) are "available", not
> > "recommended". This makes sense as they are actually harder to parse.
> >
> > Nothing stops us from being equally 'smart' in Linux too. Switch to
> > doing cheaper hypercalls whenever possible.
> >
> > Signed-off-by: Vitaly Kuznetsov <vkuzn...@redhat.com>
> > ---
>
> This is a good idea. We should probably do the same with the hypercalls for
> sending
> IPIs -- try the simpler version first and move to the more complex _EX
> version only
> if necessary.
I am not sure if this would work correctly. When I was developing the IPI
enlightenment,
what I remember was that the guest is expected to use the API recommended by
the Hypervisor.
K. Y
>
> A complication: We've recently found a problem with the code for doing IPI
> hypercalls, and the bug affects the TLB flush code as well. As secondary CPUs
> are started, there's a window of time where the hv_vp_index entry for a
> secondary CPU is uninitialized. We are seeing IPIs happening in that window,
> and
> the IPI hypercall code uses the uninitialized hv_vp_index entry. Same thing
> could
> happen with the TLB flush hypercall code. I didn't actually see any
> occurrences of
> the TLB case in my tracing, but we should fix it anyway in case a TLB flush
> gets
> added at some point in the future.
>
> KY has a patch coming. In the patch, hv_cpu_number_to_vp_number()
> and cpumask_to_vpset() can both return U32_MAX if they encounter an
> uninitialized hv_vp_index entry, and the code needs to be able to bail out to
> the native functions for that particular IPI or TLB flush operation. Once the
> initialization of secondary CPUs is complete, the uninitialized situation
> won't
> happen again, and the hypercall path will always be used.
>
> We'll need to coordinate on these patches. Be aware that the IPI flavor of
> the
> bug is currently causing random failures when booting 4.18 RC1 on Hyper-V
> VMs
> with large vCPU counts.
>
> Reviewed-by: Michael Kelley <mikel...@microsoft.com>
