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

[Vitaly Kuznetsov]

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.
>
> 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@xxxxxxxxxxxxxxx
> Signed-off-by: Roman Kagan <rkagan@xxxxxxxxxxxxx>
> ---
> 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)). Is it actually correct? (I think you've
tested this and it is but) Michael, could you please shed some light
here?

> +
> +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? 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?

> +#ifdef CONFIG_X86_X2APIC
> +		}
> +#endif
>  		apic->icr_read  = hv_apic_icr_read;
>  	}
>  }

-- 
Vitaly