linux-kernel - Re: [PATCH] RISC-V: KVM: Remove automatic I/O mapping for VM

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Message-ID: <CAJF2gTS-qZDgxsqxr7OjZijwxc4GY2MKCabMbE3wvtzx0TDixQ@mail.gmail.com>
Date: Fri, 24 Oct 2025 03:03:23 -0700
From: Guo Ren <guoren@...nel.org>
To: Anup Patel <anup@...infault.org>
Cc: fangyu.yu@...ux.alibaba.com, atish.patra@...ux.dev, pjw@...nel.org, 
	palmer@...belt.com, aou@...s.berkeley.edu, alex@...ti.fr, pbonzini@...hat.com, 
	jiangyifei@...wei.com, kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, jgg@...dia.com, 
	alex.williamson@...hat.com
Subject: Re: [PATCH] RISC-V: KVM: Remove automatic I/O mapping for VM_PFNMAP

On Fri, Oct 24, 2025 at 12:32 AM Anup Patel <anup@...infault.org> wrote:
>
> On Tue, Oct 21, 2025 at 8:58 PM Guo Ren <guoren@...nel.org> wrote:
> >
> > On Mon, Oct 20, 2025 at 6:08 AM <fangyu.yu@...ux.alibaba.com> wrote:
> > >
> > > From: Fangyu Yu <fangyu.yu@...ux.alibaba.com>
> > >
> > > As of commit aac6db75a9fc ("vfio/pci: Use unmap_mapping_range()"),
> > > vm_pgoff may no longer guaranteed to hold the PFN for VM_PFNMAP
> > > regions. Using vma->vm_pgoff to derive the HPA here may therefore
> > > produce incorrect mappings.
> > >
> > > Instead, I/O mappings for such regions can be established on-demand
> > > during g-stage page faults, making the upfront ioremap in this path
> > > is unnecessary.
> > >
> > > Fixes: 9d05c1fee837 ("RISC-V: KVM: Implement stage2 page table programming")
> > The Fixes tag should be 'commit aac6db75a9fc ("vfio/pci: Use
> > unmap_mapping_range()")'.
> >
> > A stable tree necessitates minimizing the "Fixes tag" interference.
> >
> > We also need to
> > Cc: Jason Gunthorpe <jgg@...dia.com>
> > Cc: Alex Williamson <alex.williamson@...hat.com>
> > For review.
> >
> > > Signed-off-by: Fangyu Yu <fangyu.yu@...ux.alibaba.com>
> > > ---
> > >  arch/riscv/kvm/mmu.c | 20 +-------------------
> > >  1 file changed, 1 insertion(+), 19 deletions(-)
> > >
> > > diff --git a/arch/riscv/kvm/mmu.c b/arch/riscv/kvm/mmu.c
> > > index 525fb5a330c0..84c04c8f0892 100644
> > > --- a/arch/riscv/kvm/mmu.c
> > > +++ b/arch/riscv/kvm/mmu.c
> > > @@ -197,8 +197,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
> > >
> > >         /*
> > >          * A memory region could potentially cover multiple VMAs, and
> > > -        * any holes between them, so iterate over all of them to find
> > > -        * out if we can map any of them right now.
> > > +        * any holes between them, so iterate over all of them.
> > >          *
> > >          *     +--------------------------------------------+
> > >          * +---------------+----------------+   +----------------+
> > > @@ -229,32 +228,15 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
> > >                 vm_end = min(reg_end, vma->vm_end);
> > >
> > >                 if (vma->vm_flags & VM_PFNMAP) {
> > > -                       gpa_t gpa = base_gpa + (vm_start - hva);
> > > -                       phys_addr_t pa;
> > > -
> > > -                       pa = (phys_addr_t)vma->vm_pgoff << PAGE_SHIFT;
> > > -                       pa += vm_start - vma->vm_start;
> > > -
> > >                         /* IO region dirty page logging not allowed */
> > >                         if (new->flags & KVM_MEM_LOG_DIRTY_PAGES) {
> > >                                 ret = -EINVAL;
> > >                                 goto out;
> > >                         }
> > > -
> > > -                       ret = kvm_riscv_mmu_ioremap(kvm, gpa, pa, vm_end - vm_start,
> > > -                                                   writable, false);
> > > -                       if (ret)
> > > -                               break;
> > Defering the ioremap to the g-stage page fault looks good to me, as it
> > simplifies the implementation here.
> >
> > Acked-by: Guo Ren <guoren@...nel.org>
>
> I think you meant Reviewed-by and not Acked-by.
Yes,

Reviewed-by: Guo Ren <guoren@...nel.org>

>
> I have updated the Fixes tag at the time of merging.
Okay.

>
> Regards,
> Anup



-- 
Best Regards
 Guo Ren