linux-kernel - Re: [PATCH v3 03/28] drm/i915/gvt: Verify hugepages are contiguous in physical address space

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Date:   Thu, 18 May 2023 17:06:43 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     Paolo Bonzini <pbonzini@...hat.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Zhi Wang <zhi.a.wang@...el.com>, <kvm@...r.kernel.org>,
        <intel-gvt-dev@...ts.freedesktop.org>,
        <intel-gfx@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v3 03/28] drm/i915/gvt: Verify hugepages are contiguous
 in physical address space

On Wed, May 17, 2023 at 07:50:26AM -0700, Sean Christopherson wrote:
> On Tue, May 16, 2023, Yan Zhao wrote:
> > hi Sean
> > 
> > Do you think it's necessary to double check that struct page pointers
> > are also contiguous?
> 
> No, the virtual address space should be irrelevant.  The only way it would be
> problematic is if something in dma_map_page() expected to be able to access the
> entire chunk of memory by getting the virtual address of only the first page,
> but I can't imagine that code is reading or writing memory, let alone doing so
> across a huge range of memory.
Yes, I do find arm_iommu version of dma_map_page() access the memory by getting
virtual address of pages passed in, but it's implemented as page by page, not only
from the first page.

dma_map_page
  dma_map_page_attrs
    ops->map_page
      arm_iommu_map_page
         __dma_page_cpu_to_dev
           dma_cache_maint_page


Just a little worried about the condition of PFNs are contiguous
while they belong to different backends, e.g. one from system memory and
one from MMIO.
But I don't know how to avoid this without complicated checks.
And this condition might not happen in practice.


> 
> > And do you like to also include a fix as below, which is to remove the
> > warning in vfio_device_container_unpin_pages() when npage is 0?
> > 
> > @ -169,7 +173,8 @@ static int gvt_pin_guest_page(struct intel_vgpu *vgpu, unsigned long gfn,
> >         *page = base_page;
> >         return 0;
> >  err:
> > -       gvt_unpin_guest_page(vgpu, gfn, npage * PAGE_SIZE);
> > +       if (npage)
> > +               gvt_unpin_guest_page(vgpu, gfn, npage * PAGE_SIZE);
> >         return ret;
> >  }
> 
> Sure.  Want to give your SoB?  I'll write a changelog.
>
Thanks!
It's just a small code piece. Whatever is convenient for you :)