linux-kernel - Re: `AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT=y` causes AMDGPU to fail on Ryzen: amdgpu: SME is not compatible with RAVEN

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Message-ID: <YV1vcKpRvF9WTwAo@zn.tnic>
Date:   Wed, 6 Oct 2021 11:42:08 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Alex Deucher <alexdeucher@...il.com>
Cc:     Paul Menzel <pmenzel@...gen.mpg.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>
Subject: Re: `AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT=y` causes AMDGPU to fail on
 Ryzen: amdgpu: SME is not compatible with RAVEN

On Tue, Oct 05, 2021 at 10:48:15AM -0400, Alex Deucher wrote:
> It's not incompatible per se, but SEM requires the IOMMU be enabled
> because the C bit used for encryption is beyond the dma_mask of most
> devices.  If the C bit is not set, the en/decryption for DMA doesn't
> occur.  So you need IOMMU to be enabled in remapping mode to use SME
> with most devices.  Raven has further requirements in that it requires
> IOMMUv2 functionality to support some features which currently uses a
> direct mapping in the IOMMU and hence the C bit is not properly
> handled.

So lemme ask you this: do Raven-containing systems exist out there which
don't have IOMMUv2 functionality and which can cause boot failures when
SME is enabled in the kernel .config?

IOW, can we handle this at boot time properly, i.e., disable SME if we
detect Raven or IOMMUv2 support is missing?

If not, then we really will have to change the default.

Thx.

-- 
Regards/Gruss,
    Boris.

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