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From: Suzuki K Poulose <suzuki.poulose@arm.com>
To: Mark Brown <broonie@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 2/2] arm64: Don't use KPTI where we have E0PD
Date: Thu, 10 Oct 2019 11:24:55 +0100	[thread overview]
Message-ID: <2d53ed87-34f3-33e1-5501-77895662c47a@arm.com> (raw)
In-Reply-To: <20191009175205.GN2036@sirena.org.uk>

Hi Mark

On 09/10/2019 18:52, Mark Brown wrote:
> On Tue, Sep 24, 2019 at 10:13:18AM +0100, Suzuki K Poulose wrote:
>> On 16/08/2019 13:10, Mark Brown wrote:
> 
>>> We definitely need some variable I think, and I think you're right that
>>> making the decision on the boot CPU would simplify things a lot.  The
> 
>> relocating the kernel. So, we may be able to perform "raw cpuid check" on
>> the boot CPU with MMU turned on, before we re-write the pagetables for KASLR
>> displacement and nG if that is needed (by maybe updating SWWAPPER_MMU_FLAGS) for
>> the boot CPU and store this information somewhere. Thus we may be able to
>> avoid another re-write of the pagetables after we have booted the secondaries.
> 
> The boot CPU is straightforward, there is only an issue on the
> secondaries where IIRC the rewrite code needs some updates as we
> get left with non-global mappings lying around.
> 
>> Discussing this with Catalin, he suggests to use a variable for the status
>> of "nG" flag for PTE/PMD_MAYBE_NG, to avoid calling the helper function
>> all the time. By using the per-CPU check we can make sure the flag is uptodate.
> 
> That was the discussion about the variable above.  We need one
> for non-optimization reasons anyway since we can't rely on
> checking the state on the current CPU.
> 
>> Also, we can continue to fail the hotplugged CPUs if we detect that the
>> pagetables are Global and the new CPU requires nG (for heterogeneous
>> systems).
> 
> There's no continuing to reject those CPUs unfortunately, we
> don't reject anything currently.  Any such systems would

In fact we do reject the hotplugged CPUs, after we have finalised
the capabilities for KPTI. So, I don't see how the behavior is different.

Cheers
Suzuki


> experience a regression when moving to a kernel where E0PD is
> enabled which doesn't seem ideal.

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  reply	other threads:[~2019-10-10 10:25 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-14 18:31 [PATCH v2 0/2] arm64: E0PD support Mark Brown
2019-08-14 18:31 ` [PATCH v2 1/2] arm64: Add initial support for E0PD Mark Brown
2019-10-10 16:13   ` Mark Rutland
2019-10-11 11:17     ` Mark Brown
2019-10-11 11:40       ` Will Deacon
2019-10-11 12:57         ` Mark Rutland
2019-10-11 12:58         ` Catalin Marinas
2019-10-11 13:46         ` Mark Brown
2019-08-14 18:31 ` [PATCH v2 2/2] arm64: Don't use KPTI where we have E0PD Mark Brown
2019-08-15 16:35   ` Will Deacon
2019-08-15 18:00     ` Mark Brown
2019-08-16 11:31       ` Mark Brown
2019-08-16 10:24     ` Catalin Marinas
2019-08-16 12:10       ` Mark Brown
2019-09-24  9:13         ` Suzuki K Poulose
2019-10-09 17:52           ` Mark Brown
2019-10-10 10:24             ` Suzuki K Poulose [this message]
2019-10-10 16:04               ` Mark Brown

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