linux-kernel - Re: [PATCH v3 1/1] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags

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Message-ID: <20250423130323.GE1648741@nvidia.com>
Date: Wed, 23 Apr 2025 10:03:23 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Oliver Upton <oliver.upton@...ux.dev>,
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Subject: Re: [PATCH v3 1/1] KVM: arm64: Allow cacheable stage 2 mapping using
 VMA flags

On Wed, Apr 23, 2025 at 01:26:51PM +0100, Catalin Marinas wrote:

> Ah, good point. So we want a strict cacheable mapping even if there is
> no user/VMM mapping alias. Is it because KVM cannot do cache maintenance
> on the PFNMAP or because the device does not support other memory types?
> Both valid reasons though.

Lack of cache maintenance is the main problem for the vfio-grace device

> In this case KVM still needs to know the properties of the device. Not
> sure how it could best do this without a vma.

Well, the idea I hope to succeed with would annotate this kind of
information inside the page list that would be exchanged through the
FD.

There is an monstrous thread here about this topic:

https://lore.kernel.org/all/20250107142719.179636-1-yilun.xu@linux.intel.com/

I can't find it in the huge thread but I did explain some thoughts on
how it could work

Jason