linux-kernel - Re: [PATCH v2 1/1] KVM: arm64: Allow cacheable stage 2 mapping using VMA flags

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Message-ID: <20250106165159.GJ5556@nvidia.com>
Date: Mon, 6 Jan 2025 12:51:59 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: ankita@...dia.com, maz@...nel.org, oliver.upton@...ux.dev,
	joey.gouly@....com, suzuki.poulose@....com, yuzenghui@...wei.com,
	catalin.marinas@....com, will@...nel.org, ryan.roberts@....com,
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	cjia@...dia.com, kwankhede@...dia.com, targupta@...dia.com,
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	alex.williamson@...hat.com, sebastianene@...gle.com,
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	ardb@...nel.org, akpm@...ux-foundation.org, gshan@...hat.com,
	linux-mm@...ck.org, kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/1] KVM: arm64: Allow cacheable stage 2 mapping using
 VMA flags

On Fri, Dec 20, 2024 at 04:42:35PM +0100, David Hildenbrand wrote:
> On 18.11.24 14:19, ankita@...dia.com wrote:
> > From: Ankit Agrawal <ankita@...dia.com>
> > 
> > Currently KVM determines if a VMA is pointing at IO memory by checking
> > pfn_is_map_memory(). However, the MM already gives us a way to tell what
> > kind of memory it is by inspecting the VMA.
> 
> Do you primarily care about VM_PFNMAP/VM_MIXEDMAP VMAs, or also other VMA
> types?

I think this is exclusively about allowing cachable memory inside a
VM_PFNMAP VMA (created by VFIO) remain cachable inside the guest VM.

> > This patch solves the problems where it is possible for the kernel to
> > have VMAs pointing at cachable memory without causing
> > pfn_is_map_memory() to be true, eg DAX memremap cases and CXL/pre-CXL
> > devices. This memory is now properly marked as cachable in KVM.
> 
> Does this only imply in worse performance, or does this also affect
> correctness? I suspect performance is the problem, correct?

Correctness. Things like atomics don't work on non-cachable mappings.

> Maybe one could just reject such cases (if KVM PFN lookup code not
> already rejects them, which might just be that case IIRC).

At least VFIO enforces SHARED or it won't create the VMA.

drivers/vfio/pci/vfio_pci_core.c:       if ((vma->vm_flags & VM_SHARED) == 0)

This is pretty normal/essential for drivers..

Are you suggesting the VMA flags should be inspected more?
VM_SHARED/PFNMAP before allowing this?

Jason