linux-kernel - Re: [PATCH v6] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests.

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Message-ID: <20201124093837.GD4009@zn.tnic>
Date:   Tue, 24 Nov 2020 10:38:37 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Kalra, Ashish" <Ashish.Kalra@....com>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        "hch@....de" <hch@....de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "dave.hansen@...ux-intel.com" <dave.hansen@...ux-intel.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Singh, Brijesh" <brijesh.singh@....com>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        "Grimm, Jon" <Jon.Grimm@....com>,
        "rientjes@...gle.com" <rientjes@...gle.com>
Subject: Re: [PATCH v6] swiotlb: Adjust SWIOTBL bounce buffer size for SEV
 guests.

On Tue, Nov 24, 2020 at 09:25:06AM +0000, Kalra, Ashish wrote:
> But what will be the criteria to figure out this percentage?
>
> As I mentioned earlier, this can be made as complicated as possible by
> adding all kind of heuristics but without any predictable performance
> gain.
>
> Or it can be kept simple by using a static percentage value.

Yes, static percentage number based on the guest memory. X% of the guest
memory is used for SWIOTLB.

Since you use sev_active(), it means the size computation is done in the
guest so that SWIOTLB size is per-guest. Yes?

If so, you can simply take, say, 5% of the guest memory's size and use
that for SWIOTLB buffers. Or 6 or X or whatever.

Makes sense?

-- 
Regards/Gruss,
    Boris.

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