Re: [PATCH v15 03/20] arm64: Convert hcalls to use HVC immediate value

From: Dave Martin <Dave.Martin@arm.com>
To: Geoff Levand <geoff@infradead.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	marc.zyngier@arm.com, Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	AKASHI Takahiro <takahiro.akashi@linaro.org>,
	James Morse <james.morse@arm.com>,
	kexec@lists.infradead.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v15 03/20] arm64: Convert hcalls to use HVC immediate value
Date: Tue, 15 Mar 2016 13:50:39 +0000	[thread overview]
Message-ID: <20160315135030.GA3701@e103592.cambridge.arm.com> (raw)
In-Reply-To: <c23696b455e99b5b111ae559c7998f3c56a664df.1457977402.git.geoff@infradead.org>

Hi

On Mon, Mar 14, 2016 at 05:48:00PM +0000, Geoff Levand wrote:
> The existing arm64 hcall implementations are limited in that they only
> allow for two distinct hcalls; with the x0 register either zero or not
> zero.  Also, the API of the hyp-stub exception vector routines and the
> KVM exception vector routines differ; hyp-stub uses a non-zero value in
> x0 to implement __hyp_set_vectors, whereas KVM uses it to implement
> kvm_call_hyp.
> 
> To allow for additional hcalls to be defined and to make the arm64 hcall
> API more consistent across exception vector routines, change the hcall
> implementations to use the 16 bit immediate value of the HVC instruction
> to specify the hcall type.

I'm a bit concerned about namespace pollution on the HVC immediate here.
Existing users tend allocate a single "random" number to identify the
API -- Xen and Jailhouse do this for example.

If we start using the HVC immediate to select functions, not just APIs,
the space is going to fill up a lot faster, if we have a multiplex
multiple APIs through it.

(We don't currently seem to multiplex APIs much here, except that we
do use HVC for PSCI calls from the guest, and it could be used for
additional paravirtualised services in the future).

> Define three new preprocessor macros HVC_CALL_HYP, HVC_GET_VECTORS, and
> HVC_SET_VECTORS to be used as hcall type specifiers and convert the
> existing __hyp_get_vectors(), __hyp_set_vectors() and kvm_call_hyp()
> routines to use these new macros when executing an HVC call.  Also,
> change the corresponding hyp-stub and KVM el1_sync exception vector
> routines to use these new macros.

It would also be preferable to keep the 32-bit and 64-bit APIs the same;
we should avoid having them different unless there's a clinching
technical reason...

There may be some historical context for this that I'm missing...

Cheers
---Dave

> 
> Signed-off-by: Geoff Levand <geoff@infradead.org>
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
>  arch/arm64/include/asm/virt.h  | 27 +++++++++++++++++++++++++++
>  arch/arm64/kernel/hyp-stub.S   | 32 +++++++++++++++++++++-----------
>  arch/arm64/kvm/hyp.S           |  3 ++-
>  arch/arm64/kvm/hyp/hyp-entry.S |  9 ++++++---
>  4 files changed, 56 insertions(+), 15 deletions(-)
> 

[...]

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