From: Marc Zyngier <maz@kernel.org>
To: Mark Brown <broonie@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
Sudeep Holla <sudeep.holla@arm.com>,
Ard Biesheuvel <ardb@kernel.org>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v5] arm64: smccc: Support SMCCC v1.3 SVE register saving hint
Date: Tue, 01 Jun 2021 14:58:29 +0100 [thread overview]
Message-ID: <87mts91ybe.wl-maz@kernel.org> (raw)
In-Reply-To: <20210601130419.5432-1-broonie@kernel.org>
On Tue, 01 Jun 2021 14:04:19 +0100,
Mark Brown <broonie@kernel.org> wrote:
>
> SMCCC v1.2 requires that all SVE state be preserved over SMC calls which
> introduces substantial overhead in the common case where there is no SVE
> state in the registers. To avoid this SMCCC v1.3 introduces a flag which
> allows the caller to say that there is no state that needs to be preserved
> in the registers. Make use of this flag, setting it if the SMCCC version
> indicates support for it and the TIF_ flags indicate that there is no live
> SVE state in the registers, this avoids placing any constraints on when
> SMCCC calls can be done or triggering extra saving and reloading of SVE
> register state in the kernel.
>
> This would be straightforward enough except for the rather entertaining
> inline assembly we use to do SMCCC v1.1 calls to allow us to take advantage
> of the limited number of registers it clobbers. Deal with this by having a
> function which we call immediately before issuing the SMCCC call to make
> our checks and set the flag. Using alternatives the overhead if SVE is
> supported but not detected at runtime can be reduced to a single NOP.
>
> Signed-off-by: Mark Brown <broonie@kernel.org>
> Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
M.
--
Without deviation from the norm, progress is not possible.
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next prev parent reply other threads:[~2021-06-01 14:00 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-01 13:04 [PATCH v5] arm64: smccc: Support SMCCC v1.3 SVE register saving hint Mark Brown
2021-06-01 13:58 ` Marc Zyngier [this message]
2021-06-01 18:20 ` Will Deacon
2021-06-01 18:32 ` Will Deacon
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