linux-kernel - Re: [PATCH v2 1/1] iommu/sva: Invalidate KVA range on kernel TLB flush

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Message-ID: <2a1ffe30-b0a6-45b3-8dcb-feaa285e1e5b@linux.intel.com>
Date: Thu, 24 Jul 2025 11:01:12 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Yu Zhang <zhangyu1@...ux.microsoft.com>,
 Dave Hansen <dave.hansen@...el.com>
Cc: Jason Gunthorpe <jgg@...dia.com>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
 Kevin Tian <kevin.tian@...el.com>, Jann Horn <jannh@...gle.com>,
 Vasant Hegde <vasant.hegde@....com>, Alistair Popple <apopple@...dia.com>,
 Peter Zijlstra <peterz@...radead.org>, Uladzislau Rezki <urezki@...il.com>,
 Jean-Philippe Brucker <jean-philippe@...aro.org>,
 Andy Lutomirski <luto@...nel.org>, "Tested-by : Yi Lai" <yi1.lai@...el.com>,
 iommu@...ts.linux.dev, security@...nel.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH v2 1/1] iommu/sva: Invalidate KVA range on kernel TLB
 flush

On 7/11/25 16:17, Yu Zhang wrote:
> On Thu, Jul 10, 2025 at 08:26:06AM -0700, Dave Hansen wrote:
>> On 7/10/25 06:22, Jason Gunthorpe wrote:
>>>> Why does this matter? We flush the CPU TLB in a bunch of different ways,
>>>> _especially_ when it's being done for kernel mappings. For example,
>>>> __flush_tlb_all() is a non-ranged kernel flush which has a completely
>>>> parallel implementation with flush_tlb_kernel_range(). Call sites that
>>>> use_it_ are unaffected by the patch here.
>>>>
>>>> Basically, if we're only worried about vmalloc/vfree freeing page
>>>> tables, then this patch is OK. If the problem is bigger than that, then
>>>> we need a more comprehensive patch.
>>> I think we are worried about any place that frees page tables.
>> The two places that come to mind are the remove_memory() code and
>> __change_page_attr().
>>
>> The remove_memory() gunk is in arch/x86/mm/init_64.c. It has a few sites
>> that do flush_tlb_all(). Now that I'm looking at it, there look to be
>> some races between freeing page tables pages and flushing the TLB. But,
>> basically, if you stick to the sites in there that do flush_tlb_all()
>> after free_pagetable(), you should be good.
>>
>> As for the __change_page_attr() code, I think the only spot you need to
>> hit is cpa_collapse_large_pages() and maybe the one in
>> __split_large_page() as well.
>>
>> This is all disturbingly ad-hoc, though. The remove_memory() code needs
>> fixing and I'll probably go try to bring some order to the chaos in the
>> process of fixing it up. But that's a separate problem than this IOMMU fun.
>>
> Could we consider to split the flush_tlb_kernel_range() into 2 different
> versions:
> - the one which only flushes the CPU TLB
> - the one which flushes the CPU paging structure cache and then notifies
>    IOMMU to do the same(e.g., in pud_free_pmd_page()/pmd_free_pte_page())?

 From the perspective of an IOMMU, there is no need to split. IOMMU SVA
only allows the device to access user-space memory with user
permission. Access to kernel address space with privileged permission
is not allowed. Therefore, the IOMMU subsystem only needs a callback to
invalidate the paging structure cache.

Thanks,
baolu