RE: [PATCH v3] x86/hyperv: make vapic support x2apic mode

[Michael Kelley]

From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> Sent: Wednesday, October 9, 2019 8:27 AM
> 
> Roman Kagan <rkagan@xxxxxxxxxxxxx> writes:
> 
> > Now that there's Hyper-V IOMMU driver, Linux can switch to x2apic mode
> > when supported by the vcpus.
> >
> > However, the apic access functions for Hyper-V enlightened apic assume
> > xapic mode only.
> >
> > As a result, Linux fails to bring up secondary cpus when run as a guest
> > in QEMU/KVM with both hv_apic and x2apic enabled.
> >
> > According to Michael Kelley, when in x2apic mode, the Hyper-V synthetic
> > apic MSRs behave exactly the same as the corresponding architectural
> > x2apic MSRs, so there's no need to override the apic accessors.  The
> > only exception is hv_apic_eoi_write, which benefits from lazy EOI when
> > available; however, its implementation works for both xapic and x2apic
> > modes.
> >
> > Fixes: 29217a474683 ("iommu/hyper-v: Add Hyper-V stub IOMMU driver")
> > Fixes: 6b48cb5f8347 ("X86/Hyper-V: Enlighten APIC access")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Suggested-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
> > Signed-off-by: Roman Kagan <rkagan@xxxxxxxxxxxxx>
> > ---
> > v2 -> v3:
> > - do not introduce x2apic-capable hv_apic accessors; leave original
> >   x2apic accessors instead
> >
> > v1 -> v2:
> > - add ifdefs to handle !CONFIG_X86_X2APIC
> >
> >  arch/x86/hyperv/hv_apic.c | 17 +++++++++++++----
> >  1 file changed, 13 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/hyperv/hv_apic.c b/arch/x86/hyperv/hv_apic.c
> > index 5c056b8aebef..26eeff5bd535 100644
> > --- a/arch/x86/hyperv/hv_apic.c
> > +++ b/arch/x86/hyperv/hv_apic.c
> > @@ -261,10 +261,19 @@ void __init hv_apic_init(void)
> >
> >  	if (ms_hyperv.hints & HV_X64_APIC_ACCESS_RECOMMENDED) {
> >  		pr_info("Hyper-V: Using MSR based APIC access\n");
> 
> This pr_info() becomes a bit misleading in x2apic mode, maybe do
> something like
> 
> pr_info("Hyper-V: using Enlightened APIC (%s mode)",
>         x2apic_enabled() ? "x2apic" : "xapic");

Yes, I like this.  But tweak the capitalization of the message:

pr_info("Hyper-V: Using enlightened APIC (%s mode)",

> 
> > +		/*
> > +		 * With x2apic, architectural x2apic MSRs are equivalent to the
> > +		 * respective synthetic MSRs, so there's no need to override
> > +		 * the apic accessors.  The only exception is
> > +		 * hv_apic_eoi_write, because it benefits from lazy EOI when
> > +		 * available, but it works for both xapic and x2apic modes.
> > +		 */
> >  		apic_set_eoi_write(hv_apic_eoi_write);
> > -		apic->read      = hv_apic_read;
> > -		apic->write     = hv_apic_write;
> > -		apic->icr_write = hv_apic_icr_write;
> > -		apic->icr_read  = hv_apic_icr_read;
> > +		if (!x2apic_enabled()) {
> > +			apic->read      = hv_apic_read;
> > +			apic->write     = hv_apic_write;
> > +			apic->icr_write = hv_apic_icr_write;
> > +			apic->icr_read  = hv_apic_icr_read;
> > +		}
> >  	}
> >  }
> 
> Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> 
Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>