Re: [PATCH v3] 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.
>
> 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");

> +		/*
> +		 * 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>

-- 
Vitaly