linux-kernel - Re: [PATCH v3 2/2] kvm: arm64: set io memory s2 pte as normalnc for vfio pci devices

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Date:   Wed, 13 Dec 2023 20:05:29 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Catalin Marinas <catalin.marinas@....com>, ankita@...dia.com,
        maz@...nel.org, suzuki.poulose@....com, yuzenghui@...wei.com,
        will@...nel.org, alex.williamson@...hat.com, kevin.tian@...el.com,
        yi.l.liu@...el.com, ardb@...nel.org, akpm@...ux-foundation.org,
        gshan@...hat.com, linux-mm@...ck.org, lpieralisi@...nel.org,
        aniketa@...dia.com, cjia@...dia.com, kwankhede@...dia.com,
        targupta@...dia.com, vsethi@...dia.com, acurrid@...dia.com,
        apopple@...dia.com, jhubbard@...dia.com, danw@...dia.com,
        mochs@...dia.com, kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 2/2] kvm: arm64: set io memory s2 pte as normalnc for
 vfio pci devices

Hi,

Sorry, a bit late to the discussion :)

On Tue, Dec 12, 2023 at 02:11:56PM -0400, Jason Gunthorpe wrote:
> On Tue, Dec 12, 2023 at 05:46:34PM +0000, Catalin Marinas wrote:
> > should know the implications. There's also an expectation that the
> > actual driver (KVM guests) or maybe later DPDK can choose the safe
> > non-cacheable or write-combine (Linux terminology) attributes for the
> > BAR.
> 
> DPDK won't rely on this interface

Wait, so what's the expected interface for determining the memory
attributes at stage-1? I'm somewhat concerned that we're conflating two
things here:

 1) KVM needs to know the memory attributes to use at stage-2, which
    isn't fundamentally different from what's needed for userspace
    stage-1 mappings.

 2) KVM additionally needs a hint that the device / VFIO can handle
    mismatched aliases w/o the machine exploding. This goes beyond
    supporting Normal-NC mappings at stage-2 and is really a bug
    with our current scheme (nGnRnE at stage-1, nGnRE at stage-2).

I was hoping that (1) could be some 'common' plumbing for both userspace
and KVM mappings. And for (2), any case where a device is intolerant of
mismatches && KVM cannot force the memory attributes should be rejected.

AFAICT, the only reason PCI devices can get the blanket treatment of
Normal-NC at stage-2 is because userspace has a Device-* mapping and can't
speculatively load from the alias. This feels a bit hacky, and maybe we
should prioritize an interface for mapping a device into a VM w/o a
valid userspace mapping.

I very much understand that this has been going on for a while, and we
need to do *something* to get passthrough working well for devices that
like 'WC'. I just want to make sure we don't paint ourselves into a corner
that's hard to get out of in the future.

-- 
Thanks,
Oliver