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Date:   Wed, 6 Dec 2023 11:16:03 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Marc Zyngier <maz@kernel.org>, ankita@nvidia.com,
        Shameerali Kolothum Thodi 
        <shameerali.kolothum.thodi@huawei.com>, oliver.upton@linux.dev,
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Subject: Re: [PATCH v2 1/1] KVM: arm64: allow the VM to select DEVICE_* and
 NORMAL_NC for IO memory
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On Wed, Dec 06, 2023 at 12:14:18PM +0000, Catalin Marinas wrote:

> We could do with a pgprot_maybewritecombine() or
> pgprot_writecombinenospec() (similar to Jason's idea but without
> changing the semantics of pgprot_device()). For the user mapping on
> arm64 this would be Device (even _GRE) since it can't disable
> speculation but stage 2 would leave the decision to the guest since the
> speculative loads aren't much different from committed loads done
> wrongly.

This would be fine, as would a VMA flag. Please pick one :)

I think a VMA flag is simpler than messing with pgprot.

> If we want the VMM to drive this entirely, we could add a new mmap()
> flag like MAP_WRITECOMBINE or PROT_WRITECOMBINE. They do feel a bit

As in the other thread, we cannot unconditionally map NORMAL_NC into
the VMM.

> The latter has some benefits for DPDK but it's a lot more involved
> with

DPDK WC support will be solved with some VFIO-only change if anyone
ever cares to make it, if that is what you mean.

> having to add device-specific knowledge into the VMM. The VMM would also
> have to present the whole BAR contiguously to the guest even if there
> are different mapping attributes within the range. So a lot of MAP_FIXED
> uses. I'd rather leaving this decision with the guest than the VMM, it
> looks like more hassle to create those mappings. The VMM or the VFIO
> could only state write-combine and speculation allowed.

We talked about this already, the guest must decide, the VMM doesn't
have the information to pre-predict which pages the guest will want to
use WC on.

Jason