RE: [PATCH] x86/hyper-v: use cheaper HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE} hypercalls when possible

[KY Srinivasan]

> -----Original Message-----
> From: Michael Kelley (EOSG)
> Sent: Tuesday, June 19, 2018 10:57 AM
> To: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>; x86@xxxxxxxxxx
> Cc: devel@xxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; KY
> Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang
> <haiyangz@xxxxxxxxxxxxx>; Stephen Hemminger
> <sthemmin@xxxxxxxxxxxxx>; Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Ingo
> Molnar <mingo@xxxxxxxxxx>; H. Peter Anvin <hpa@xxxxxxxxx>; Tianyu Lan
> <Tianyu.Lan@xxxxxxxxxxxxx>
> Subject: RE: [PATCH] x86/hyper-v: use cheaper
> HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE} hypercalls when possible
> 
> > -----Original Message-----
> > From: linux-kernel-owner@xxxxxxxxxxxxxxx <linux-kernel-
> owner@xxxxxxxxxxxxxxx> On Behalf
> > Of Vitaly Kuznetsov
> > Sent: Friday, June 15, 2018 9:30 AM
> > To: x86@xxxxxxxxxx
> > Cc: devel@xxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; KY
> Srinivasan
> > <kys@xxxxxxxxxxxxx>; Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>;
> Stephen Hemminger
> > <sthemmin@xxxxxxxxxxxxx>; Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Ingo
> Molnar
> > <mingo@xxxxxxxxxx>; H. Peter Anvin <hpa@xxxxxxxxx>; Tianyu Lan
> > <Tianyu.Lan@xxxxxxxxxxxxx>
> > 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 <vkuznets@xxxxxxxxxx>
> > ---
> 
> 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 <mikelley@xxxxxxxxxxxxx>