Re: [RFC 0/6] vDSO support for Hyper-V guest on ARM64

From: Vincenzo Frascino <vincenzo.frascino@arm.com>
To: Boqun Feng <boqun.feng@gmail.com>,
	linux-hyperv@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Cc: Sasha Levin <sashal@kernel.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Michael Kelley <mikelley@microsoft.com>,
	xen-devel@lists.xenproject.org,
	Thomas Gleixner <tglx@linutronix.de>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	Will Deacon <will@kernel.org>
Subject: Re: [RFC 0/6] vDSO support for Hyper-V guest on ARM64
Date: Thu, 23 Jan 2020 10:48:07 +0000	[thread overview]
Message-ID: <ef6cb7ba-b448-cfa5-abbb-1d99d1396ce5@arm.com> (raw)
In-Reply-To: <20191216001922.23008-1-boqun.feng@gmail.com>

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Hi Boqun Feng,

sorry for the late reply.

On 16/12/2019 00:19, Boqun Feng wrote:
> Hi,
> 
> This is the RFC patchset for vDSO support in ARM64 Hyper-V guest. To
> test it, Michael's ARM64 support patchset:
> 
> 	https://lore.kernel.org/linux-arm-kernel/1570129355-16005-1-git-send-email-mikelley@microsoft.com/
> 
> is needed.
> 
> Similar as x86, Hyper-V on ARM64 use a TSC page for guests to read
> the virtualized hardware timer, this TSC page is read-only for the
> guests, so could be used for vDSO data page. And the vDSO (userspace)
> code could use the same code for timer reading as kernel, since
> they read the same TSC page.
> 

I had a look to your patches and overall, I could not understand why we can't
use the arch_timer to do the same things you are doing with the one you
introduced in this series. What confuses me is that KVM works just fine with the
arch_timer which was designed with virtualization in mind. Why do we need
another one? Could you please explain?

> This patchset therefore extends ARM64's __vsdo_init() to allow multiple
> data pages and introduces the vclock_mode concept similar to x86 to
> allow different platforms (bare-metal, Hyper-V, etc.) to switch to
> different __arch_get_hw_counter() implementations. The rest of this
> patchset does the necessary setup for Hyper-V guests: mapping tsc page,
> enabling userspace to read cntvct, etc. to enable vDSO.
> 
> This patchset consists of 6 patches:
> 
> patch #1 allows hv_get_raw_timer() definition to be overridden for
> userspace and kernel to share the same hv_read_tsc_page() definition.
> 
> patch #2 extends ARM64 to support multiple vDSO data pages.
> 
> patch #3 introduces vclock_mode similiar to x86 to allow different
> __arch_get_hw_counter() implementations for different clocksources.
> 
> patch #4 maps Hyper-V TSC page into vDSO data page.
> 
> patch #5 allows userspace to read cntvct, so that userspace can
> efficiently read the clocksource.
> 
> patch #6 enables the vDSO for ARM64 Hyper-V guest.
> 
> The whole patchset is based on v5.5-rc1 plus Michael's ARM64 support
> patchset, and I've done a few tests with:
> 
> 	https://github.com/nlynch-mentor/vdsotest
> 
> Comments and suggestions are welcome!
> 
> Regards,
> Boqun
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> 

-- 
Regards,
Vincenzo

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