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

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Message-ID: <pk2b4xgxewxotp557osucliagmziv3erepsret4hbnxnvhff2n@p2gark4kdiqw>
Date: Tue, 29 Jul 2025 01:36:44 +0800
From: Yu Zhang <zhangyu1@...ux.microsoft.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: Dave Hansen <dave.hansen@...el.com>, 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 Thu, Jul 24, 2025 at 11:01:12AM +0800, Baolu Lu wrote:
> 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. 

Indeed. That's why I was wondering if we could split flush_tlb_kernel_range()
into 2 versions - one used only after a kernal virtual address range is
unmapped, and another one used after a kernel paging structure is freed.
Only the 2nd one needs to notify the IOMMU subsystem.

B.R.
Yu