> -----Original Message----- > From: Jason Gunthorpe <[email protected]> > Sent: 07 April 2026 03:40 > To: Dan Williams <[email protected]> > Cc: Manish Honap <[email protected]>; Alex Williamson > <[email protected]>; [email protected]; > [email protected]; [email protected]; [email protected]; > [email protected]; [email protected]; [email protected]; > [email protected]; [email protected]; Yishai Hadas <[email protected]>; > Shameer Kolothum Thodi <[email protected]>; [email protected]; > Ankit Agrawal <[email protected]>; Vikram Sethi <[email protected]>; Neo > Jia <[email protected]>; Tarun Gupta (SW-GPU) <[email protected]>; Zhi > Wang <[email protected]>; Krishnakant Jaju <[email protected]>; linux- > [email protected]; [email protected]; linux- > [email protected]; [email protected] > Subject: Re: [PATCH v2 16/20] vfio/cxl: Register regions with VFIO layer > > External email: Use caution opening links or attachments > > > On Sat, Apr 04, 2026 at 12:36:53PM -0700, Dan Williams wrote: > > > Then I assume it matters that memremap() sometimes silently falls back > > to the direct map. The "VFIO owns" expectation needs to guard against > > some helpful platform firmware mapping accelerator memory as System RAM. > > I don't think how memremap works under the covers matters to vfio, it > takes in a phys_addr_t an gives back a KVA that is cachable kernel memory. > > We just have to be mindful to not allow mismatched attributes on virtual > aliases. > > > At a minimum having VFIO fail to map in that case helps with the > > argument I have been making that "no, EFI_CONVENTIONAL_MEMORY type + > > EFI_SPECIFIC_PURPOSE flag" is not suitable for accelerators with > > private CXL memory. Those want to be enforcing "EFI_RESERVED". > > Certainly it should fail the request region if something else is using it, > so if those EFI flags plug it into the mm and it is plugged when VFIO > starts then it should stop. > > The direct map isn't really "plugged into the mm" but it does raise the > mismatched attributed issue. > > Jason Thank you all for your suggestions. As pointed out by Jason/Gregory, the whole design presented here is VFIO owned device regions + mmap + vmf_insert_pfn (patch 14) and registration at vfio_pci_open_device() (patch 16), Lazy fault (vmf_insert_pfn), zapping PTEs on reset. I am convinced on the memremap usage as discussed. VFIO owns the physical range exclusively: no struct pages are involved (vmf_insert_pfn inserts raw PFNs), and memremap(MEMREMAP_WB) gives the VMA the correct WB cache attributes. As suggested, I will add request_mem_region for memory range and a failure message if we cannot request mem region at probe. This will also catch the EFI case: if firmware marks accelerator memory as EFI_CONVENTIONAL_MEMORY, MM will claim it at boot and request_mem_region will return a failure at probe time itself. I will also add a comment in next cover letter explaining why we have decided to use memremap and not ioremap so that the decision we have taken in v2 is entirely clear at the start of review cycle. Manish
