linux-kernel - Re: [PATCH 10/19] KVM: Use follow

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Message-ID: <CAJHvVciF4riGPQBhyBwNeSWHq8m+7Zag7ewEWgLJk=VsaqKNPQ@mail.gmail.com>
Date: Fri, 9 Aug 2024 10:23:20 -0700
From: Axel Rasmussen <axelrasmussen@...gle.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Sean Christopherson <seanjc@...gle.com>, Oscar Salvador <osalvador@...e.de>, Jason Gunthorpe <jgg@...dia.com>, 
	linux-arm-kernel@...ts.infradead.org, x86@...nel.org, 
	Will Deacon <will@...nel.org>, Gavin Shan <gshan@...hat.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Zi Yan <ziy@...dia.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Catalin Marinas <catalin.marinas@....com>, Ingo Molnar <mingo@...hat.com>, 
	Alistair Popple <apopple@...dia.com>, Borislav Petkov <bp@...en8.de>, David Hildenbrand <david@...hat.com>, 
	Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Alex Williamson <alex.williamson@...hat.com>, 
	Yan Zhao <yan.y.zhao@...el.com>
Subject: Re: [PATCH 10/19] KVM: Use follow_pfnmap API

On Fri, Aug 9, 2024 at 9:09 AM Peter Xu <peterx@...hat.com> wrote:
>
> Use the new pfnmap API to allow huge MMIO mappings for VMs.  The rest work
> is done perfectly on the other side (host_pfn_mapping_level()).

I don't think it has to be done in this series, but a future
optimization to consider is having follow_pfnmap just tell the caller
about the mapping level directly. It already found this information as
part of its walk. I think there's a possibility to simplify KVM /
avoid it having to do its own walk again later.

>
>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  virt/kvm/kvm_main.c | 19 +++++++------------
>  1 file changed, 7 insertions(+), 12 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d0788d0a72cc..9fb1c527a8e1 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2862,13 +2862,11 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
>                                unsigned long addr, bool write_fault,
>                                bool *writable, kvm_pfn_t *p_pfn)
>  {
> +       struct follow_pfnmap_args args = { .vma = vma, .address = addr };
>         kvm_pfn_t pfn;
> -       pte_t *ptep;
> -       pte_t pte;
> -       spinlock_t *ptl;
>         int r;
>
> -       r = follow_pte(vma, addr, &ptep, &ptl);
> +       r = follow_pfnmap_start(&args);
>         if (r) {
>                 /*
>                  * get_user_pages fails for VM_IO and VM_PFNMAP vmas and does
> @@ -2883,21 +2881,19 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
>                 if (r)
>                         return r;
>
> -               r = follow_pte(vma, addr, &ptep, &ptl);
> +               r = follow_pfnmap_start(&args);
>                 if (r)
>                         return r;
>         }
>
> -       pte = ptep_get(ptep);
> -
> -       if (write_fault && !pte_write(pte)) {
> +       if (write_fault && !args.writable) {
>                 pfn = KVM_PFN_ERR_RO_FAULT;
>                 goto out;
>         }
>
>         if (writable)
> -               *writable = pte_write(pte);
> -       pfn = pte_pfn(pte);
> +               *writable = args.writable;
> +       pfn = args.pfn;
>
>         /*
>          * Get a reference here because callers of *hva_to_pfn* and
> @@ -2918,9 +2914,8 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
>          */
>         if (!kvm_try_get_pfn(pfn))
>                 r = -EFAULT;
> -
>  out:
> -       pte_unmap_unlock(ptep, ptl);
> +       follow_pfnmap_end(&args);
>         *p_pfn = pfn;
>
>         return r;
> --
> 2.45.0
>