linux-kernel - Re: [PATCH v8] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests.

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Date:   Wed, 9 Dec 2020 13:54:42 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Ashish Kalra <ashish.kalra@....com>
Cc:     konrad.wilk@...cle.com, hch@....de, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com, x86@...nel.org, luto@...nel.org,
        peterz@...radead.org, dave.hansen@...ux-intel.com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        brijesh.singh@....com, Thomas.Lendacky@....com, Jon.Grimm@....com,
        rientjes@...gle.com
Subject: Re: [PATCH v8] swiotlb: Adjust SWIOTBL bounce buffer size for SEV
 guests.

On Wed, Dec 09, 2020 at 12:29:07PM +0000, Ashish Kalra wrote:
> As i mentioned in the main comments above, this cannot be called in
> mem_encrypt_init() as that breaks reserve_crashkernel() which depends
> on SWIOTLB buffer size

Please elaborate how does it break.

> and is called before mem_encrypt_init(), therefore, it needs to be
> called from setup_atch() before reserve_crashkernel().

I know you have your requirements what needs to be called when like all
the other vendors who want to run stuff early in a particular order but
our boot init order is a single fragile mess. So this better be done
right!

Also,

[    0.016630] software IO TLB: swiotlb_adjust:
[    0.017005] reserve_crashkernel:
[    0.050523] software IO TLB: swiotlb_init:

this looks strange - we're doing a swiotlb size adjust before init.

It probably makes sense as in: adjust the size before the SWIOTLB is
initialized so that it uses the correct size but this better be spelled
out.

> I believe that other memory encryption architectures such as s390 are
> also looking for something similar to be available.

Until you have something more palpable than belief, "let the others
extend it when they really need it." as I already mentioned.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette