Re: [PATCH v2] x86/hyperv: make vapic support x2apic mode

[Roman Kagan <rkagan@virtuozzo.com>]

On Thu, Oct 03, 2019 at 12:54:03PM +0200, Vitaly Kuznetsov wrote:
> Roman Kagan <rkagan@virtuozzo.com> 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.
> >
> > I didn't manage to make my instance of Hyper-V expose x2apic to the
> > guest; nor does Hyper-V spec document the expected behavior.  However,
> > a Windows guest running in QEMU/KVM with hv_apic and x2apic and a big
> > number of vcpus (so that it turns on x2apic mode) does use enlightened
> > apic MSRs passing unshifted 32bit destination id and falls back to the
> > regular x2apic MSRs for less frequently used apic fields.
> >
> > So implement the same behavior, by replacing enlightened apic access
> > functions (only those where it makes a difference) with their
> > x2apic-aware versions when x2apic is in use.
> >
> > Fixes: 29217a474683 ("iommu/hyper-v: Add Hyper-V stub IOMMU driver")
> > Fixes: 6b48cb5f8347 ("X86/Hyper-V: Enlighten APIC access")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
> > ---
> > v1 -> v2:
> > - add ifdefs to handle !CONFIG_X86_X2APIC
> >
> >  arch/x86/hyperv/hv_apic.c | 54 ++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 51 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/hyperv/hv_apic.c b/arch/x86/hyperv/hv_apic.c
> > index 5c056b8aebef..eb1434ae9e46 100644
> > --- a/arch/x86/hyperv/hv_apic.c
> > +++ b/arch/x86/hyperv/hv_apic.c
> > @@ -84,6 +84,44 @@ static void hv_apic_write(u32 reg, u32 val)
> >  	}
> >  }
> >  
> > +#ifdef CONFIG_X86_X2APIC
> > +static void hv_x2apic_icr_write(u32 low, u32 id)
> > +{
> > +	wrmsr(HV_X64_MSR_ICR, low, id);
> > +}
> 
> AFAIU you're trying to mirror native_x2apic_icr_write() here but this is
> different from what hv_apic_icr_write() does
> (SET_APIC_DEST_FIELD(id)).

Right.  In xapic mode the ICR2 aka the high 4 bytes of ICR is programmed
with the destination id in the highest byte; in x2apic mode the whole
ICR2 is set to the 32bit destination id.

> Is it actually correct? (I think you've tested this and it is but)

As I wrote in the commit log, I haven't tested it in the sense that I
ran a Linux guest in a Hyper-V VM exposing x2apic to the guest, because
I didn't manage to configure it to do so.  OTOH I did run a Windows
guest in QEMU/KVM with hv_apic and x2apic enabled and saw it write
destination ids unshifted to the ICR2 part of ICR, so I assume it's
correct.

> Michael, could you please shed some light here?

Would be appreciated, indeed.

> > +static u32 hv_x2apic_read(u32 reg)
> > +{
> > +	u32 reg_val, hi;
> > +
> > +	switch (reg) {
> > +	case APIC_EOI:
> > +		rdmsr(HV_X64_MSR_EOI, reg_val, hi);
> > +		return reg_val;
> > +	case APIC_TASKPRI:
> > +		rdmsr(HV_X64_MSR_TPR, reg_val, hi);
> > +		return reg_val;
> > +
> > +	default:
> > +		return native_apic_msr_read(reg);
> > +	}
> > +}
> > +
> > +static void hv_x2apic_write(u32 reg, u32 val)
> > +{
> > +	switch (reg) {
> > +	case APIC_EOI:
> > +		wrmsr(HV_X64_MSR_EOI, val, 0);
> > +		break;
> > +	case APIC_TASKPRI:
> > +		wrmsr(HV_X64_MSR_TPR, val, 0);
> > +		break;
> > +	default:
> > +		native_apic_msr_write(reg, val);
> > +	}
> > +}
> > +#endif /* CONFIG_X86_X2APIC */
> > +
> >  static void hv_apic_eoi_write(u32 reg, u32 val)
> >  {
> >  	struct hv_vp_assist_page *hvp = hv_vp_assist_page[smp_processor_id()];
> > @@ -262,9 +300,19 @@ 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");
> >  		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;
> > +#ifdef CONFIG_X86_X2APIC
> > +		if (x2apic_enabled()) {
> > +			apic->read      = hv_x2apic_read;
> > +			apic->write     = hv_x2apic_write;
> > +			apic->icr_write = hv_x2apic_icr_write;
> > +		} else {
> > +#endif
> > +			apic->read      = hv_apic_read;
> > +			apic->write     = hv_apic_write;
> > +			apic->icr_write = hv_apic_icr_write;
> 
> (just wondering): Is it always safe to assume that we cannot switch
> between apic_flat/x2apic in runtime?

I guess so.  All apic choices are made early at __init, obviously before
the secondary CPUs are brought up, and aren't reconsidered afterwards.

> Moreover, the only difference
> between hv_apic_read/hv_apic_write and hv_x2apic_read/hv_x2apic_write is
> native_apic_mem_{read,write} -> native_apic_msr_{read,write}. Would it
> make sense to move if (x2apic_enabled()) and merge these functions?

x2apic_enabled() is too heavy for that: it reads MSR_IA32_APICBASE.  One
could probably use a read-mostly global variable instead but I'm not
sure it would make the code better.

Thanks,
Roman.