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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <f13622a2-6955-48d0-9793-fba6cea97a60@redhat.com>
Date: Wed, 8 Jan 2025 17:09:13 +0100
From: David Hildenbrand <david@...hat.com>
To: Jason Gunthorpe <jgg@...dia.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,
 shahuang@...hat.com, 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, zhiw@...dia.com, mochs@...dia.com,
 udhoke@...dia.com, dnigam@...dia.com, alex.williamson@...hat.com,
 sebastianene@...gle.com, coltonlewis@...gle.com, kevin.tian@...el.com,
 yi.l.liu@...el.com, 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 06.01.25 17:51, Jason Gunthorpe wrote:
> 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.

Thanks!

> 
>>> 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.

Hah! This needs to be highlighted in the patch description. And maybe 
this even implies Fixes: etc?

> 
>> 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)

That makes a lot of sense for VFIO.

> 
> This is pretty normal/essential for drivers..
> 
> Are you suggesting the VMA flags should be inspected more?
> VM_SHARED/PFNMAP before allowing this?


I was wondering if we can safely assume that "device PFNs" can only 
exist in VM_PFNMAP mappings. Then we can avoid all that pfn_valid() / 
pfn_is_map_memory() stuff for "obviously not device" memory.

I tried to protoype, but have to give up for now; the code is too 
complicated to make quick progress :)

-- 
Cheers,

David / dhildenb