linux-kernel - Re: [PATCH v4 2/2] kvm/arm64: Try stage2 block mapping for host device MMIO

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Message-ID: <87tunyq0av.wl-maz@kernel.org>
Date:   Thu, 22 Apr 2021 07:51:36 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Gavin Shan <gshan@...hat.com>
Cc:     Keqian Zhu <zhukeqian1@...wei.com>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, wanghaibin.wang@...wei.com
Subject: Re: [PATCH v4 2/2] kvm/arm64: Try stage2 block mapping for host device MMIO

On Thu, 22 Apr 2021 03:25:23 +0100,
Gavin Shan <gshan@...hat.com> wrote:
> 
> Hi Keqian,
> 
> On 4/21/21 4:36 PM, Keqian Zhu wrote:
> > On 2021/4/21 15:52, Gavin Shan wrote:
> >> On 4/16/21 12:03 AM, Keqian Zhu wrote:
> >>> The MMIO region of a device maybe huge (GB level), try to use
> >>> block mapping in stage2 to speedup both map and unmap.
> >>> 
> >>> Compared to normal memory mapping, we should consider two more
> >>> points when try block mapping for MMIO region:
> >>> 
> >>> 1. For normal memory mapping, the PA(host physical address) and
> >>> HVA have same alignment within PUD_SIZE or PMD_SIZE when we use
> >>> the HVA to request hugepage, so we don't need to consider PA
> >>> alignment when verifing block mapping. But for device memory
> >>> mapping, the PA and HVA may have different alignment.
> >>> 
> >>> 2. For normal memory mapping, we are sure hugepage size properly
> >>> fit into vma, so we don't check whether the mapping size exceeds
> >>> the boundary of vma. But for device memory mapping, we should pay
> >>> attention to this.
> >>> 
> >>> This adds get_vma_page_shift() to get page shift for both normal
> >>> memory and device MMIO region, and check these two points when
> >>> selecting block mapping size for MMIO region.
> >>> 
> >>> Signed-off-by: Keqian Zhu <zhukeqian1@...wei.com>
> >>> ---
> >>>    arch/arm64/kvm/mmu.c | 61 ++++++++++++++++++++++++++++++++++++--------
> >>>    1 file changed, 51 insertions(+), 10 deletions(-)
> >>> 
> >>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> >>> index c59af5ca01b0..5a1cc7751e6d 100644
> >>> --- a/arch/arm64/kvm/mmu.c
> >>> +++ b/arch/arm64/kvm/mmu.c
> >>> @@ -738,6 +738,35 @@ transparent_hugepage_adjust(struct kvm_memory_slot *memslot,
> >>>        return PAGE_SIZE;
> >>>    }
> >>>    +static int get_vma_page_shift(struct vm_area_struct *vma, unsigned long hva)
> >>> +{
> >>> +    unsigned long pa;
> >>> +
> >>> +    if (is_vm_hugetlb_page(vma) && !(vma->vm_flags & VM_PFNMAP))
> >>> +        return huge_page_shift(hstate_vma(vma));
> >>> +
> >>> +    if (!(vma->vm_flags & VM_PFNMAP))
> >>> +        return PAGE_SHIFT;
> >>> +
> >>> +    VM_BUG_ON(is_vm_hugetlb_page(vma));
> >>> +
> >> 
> >> I don't understand how VM_PFNMAP is set for hugetlbfs related vma.
> >> I think they are exclusive, meaning the flag is never set for
> >> hugetlbfs vma. If it's true, VM_PFNMAP needn't be checked on hugetlbfs
> >> vma and the VM_BUG_ON() becomes unnecessary.
> > Yes, but we're not sure all drivers follow this rule. Add a BUG_ON() is
> > a way to catch issue.
> > 
> 
> I think I didn't make things clear. What I meant is VM_PFNMAP can't
> be set for hugetlbfs VMAs. So the checks here can be simplified as
> below if you agree:
> 
>     if (is_vm_hugetlb_page(vma))
>         return huge_page_shift(hstate_vma(vma));
> 
>     if (!(vma->vm_flags & VM_PFNMAP))
>         return PAGE_SHIFT;
> 
>     VM_BUG_ON(is_vm_hugetlb_page(vma));       /* Can be dropped */

No. If this case happens, I want to see it. I have explicitly asked
for it, and this check stays.

	M.

-- 
Without deviation from the norm, progress is not possible.