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

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Roman Kagan <rkagan@virtuozzo.com>,
	Michael Kelley <mikelley@microsoft.com>
Cc: "kvm\@vger.kernel.org" <kvm@vger.kernel.org>,
	Lan Tianyu <Tianyu.Lan@microsoft.com>,
	Joerg Roedel <jroedel@suse.de>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Sasha Levin <sashal@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"x86\@kernel.org" <x86@kernel.org>,
	"linux-hyperv\@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3] x86/hyperv: make vapic support x2apic mode
Date: Wed, 09 Oct 2019 17:27:06 +0200	[thread overview]
Message-ID: <87r23mx7lh.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20191009145022.28442-1-rkagan@virtuozzo.com>

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.
>
> 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@vger.kernel.org
> Suggested-by: Michael Kelley <mikelley@microsoft.com>
> Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
> ---
> 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@redhat.com>

-- 
Vitaly