linux-kernel - Re: [PATCH 5.10 38/58] KVM: arm64: Allow indirect vectors to be used without SPECTRE

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Message-ID: <ef538a31-5f73-dfc5-12a9-f5222514035c@arm.com>
Date:   Tue, 15 Mar 2022 12:20:31 +0000
From:   James Morse <james.morse@....com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Pavel Machek <pavel@...x.de>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH 5.10 38/58] KVM: arm64: Allow indirect vectors to be used
 without SPECTRE_V3A

Hi guys,

On 3/11/22 6:42 AM, Greg Kroah-Hartman wrote:
> On Fri, Mar 11, 2022 at 12:48:59AM +0100, Pavel Machek wrote:
>> What is going on here?
>>
>>> commit 5bdf3437603d4af87f9c7f424b0c8aeed2420745 upstream.
>>
>> Upstream commit 5bdf is very different from this. In particular,
>>
>>>   arch/arm64/kvm/hyp/smccc_wa.S    |   66 +++++++++++++++++++++++++++++++++++++++
>>
>> I can't find smccc_wa.S, neither in mainline, nor in -next. And it
>> looks buggy. I suspect loop_k24 should loop 24 times, but it does 8
>> loops AFAICT. Same problem with loop_k32.

Yup, that's a bug. Thanks for spotting it!
I'll post a replacement for this patch.

I only have A57 I can test this on, guess what its K value is.

> The kvm portion of these patches is the "trickiest" portions.  I'll let
> James explain them, as he did so to me when sending the backports.

KVM gets re-written fairly frequently. Earlier kernels don't have any of the infrastructure
for generating the vectors at compile time and selecting a pre-built vector at boot. Instead,
kernels of this vintage have bunch of empty vectors, and some templates they use to create
the appropriate vector at boot. See commit b881cdce77b4.
I've looked at backporting all that - its about 60 patches. I don't think its a good idea.

Thanks,

James