RE: [PATCH v5 2/8] arm64: hyperv: Add hypercall and register access functions

From: Marc Zyngier <maz@kernel.org>
To: Michael Kelley <mikelley@microsoft.com>
Cc: mark.rutland@arm.com, linux-hyperv@vger.kernel.org,
	"boqun.feng" <boqun.feng@gmail.com>,
	catalin.marinas@arm.com, jasowang@redhat.com,
	linux-kernel@vger.kernel.org, marcelo.cerri@canonical.com,
	olaf@aepfle.de, gregkh@linuxfoundation.org, apw@canonical.com,
	devel@linuxdriverproject.org, vkuznets <vkuznets@redhat.com>,
	will@kernel.org, Sunil Muthuswamy <sunilmut@microsoft.com>,
	linux-arm-kernel@lists.infradead.org
Subject: RE: [PATCH v5 2/8] arm64: hyperv: Add hypercall and register access functions
Date: Thu, 07 Nov 2019 10:19:56 +0109	[thread overview]
Message-ID: <c8403255bf874856c10f07189e27080a@www.loen.fr> (raw)
In-Reply-To: <DM5PR21MB013730D09CB8BA7658DE57F7D7790@DM5PR21MB0137.namprd21.prod.outlook.com>

On 2019-11-06 19:08, Michael Kelley wrote:
> From: Marc Zyngier <maz@kernel.org>  Sent: Wednesday, November 6,
> 2019 2:20 AM
>>
>> On 2019-10-03 20:12, Michael Kelley wrote:
>> > Add ARM64-specific code to make Hyper-V hypercalls and to
>> > access virtual processor synthetic registers via hypercalls.
>> > Hypercalls use a Hyper-V specific calling sequence with a non-zero
>> > immediate value per Section 2.9 of the SMC Calling Convention
>> > spec.
>>
>> I find this "following the spec by actively sidestepping it" counter
>> productive. You (or rather the Hyper-V people) are reinventing the
>> wheel (of the slightly square variety) instead of using the standard
>> that the whole of the ARM ecosystem seems happy to take advantage
>> of.
>>
>> I wonder what is the rational for this. If something doesn't quite
>> work for Hyper-V, I think we'd all like to know.
>>
>
> I'll go another round internally with the Hyper-V people on this
> topic and impress upon them the desire of the Linux community to
> have Hyper-V adopt the true spirit of the spec.  But I know they are
> fairly set in their approach at this point, regardless of the 
> technical
> merits or lack thereof.  Hyper-V is shipping and in use as a 
> commercial
> product on ARM64 hardware, which makes it harder to change.  I
> hope we can find a way to avoid a complete impasse ....

Hyper-V shipping with their own calling convention is fine by me. Linux
having to implement multiple calling conventions because the Hyper-V
folks refuse (for undisclosed reason) to adopt the standard isn't fine 
at
all.

HV can perfectly retain its interface for Windows or other things, but
please *at least* implement the standard interface on which all 
existing
operating systems rely.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...
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