linux-kernel - Re: [PATCH 3/3] kvm: add a function to check if page is from NVDIMM pmem.

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Date:   Wed, 4 Jul 2018 17:25:44 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Zhang Yi <yi.z.zhang@...ux.intel.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        dan.j.williams@...el.com, jack@...e.cz, hch@....de,
        yu.c.zhang@...el.com
Cc:     linux-mm@...ck.org, rkrcmar@...hat.com, yi.z.zhang@...el.com
Subject: Re: [PATCH 3/3] kvm: add a function to check if page is from NVDIMM
 pmem.

On 04/07/2018 17:30, Zhang Yi wrote:
> For device specific memory space, when we move these area of pfn to
> memory zone, we will set the page reserved flag at that time, some of
> these reserved for device mmio, and some of these are not, such as
> NVDIMM pmem.
> 
> Now, we map these dev_dax or fs_dax pages to kvm for DIMM/NVDIMM
> backend, since these pages are reserved. the check of
> kvm_is_reserved_pfn() misconceives those pages as MMIO. Therefor, we
> introduce 2 page map types, MEMORY_DEVICE_FS_DAX/MEMORY_DEVICE_DEV_DAX,
> to indentify these pages are from NVDIMM pmem. and let kvm treat these
> as normal pages.
> 
> Without this patch, Many operations will be missed due to this
> mistreatment to pmem pages. For example, a page may not have chance to
> be unpinned for KVM guest(in kvm_release_pfn_clean); not able to be
> marked as dirty/accessed(in kvm_set_pfn_dirty/accessed) etc.
> 
> Signed-off-by: Zhang Yi <yi.z.zhang@...ux.intel.com>
> Signed-off-by: Zhang Yu <yu.c.zhang@...ux.intel.com>
> ---
>  virt/kvm/kvm_main.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index afb2e6e..1365d18 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -140,10 +140,23 @@ __weak void kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,
>  {
>  }
>  
> +static bool kvm_is_nd_pfn(kvm_pfn_t pfn)
> +{
> +	struct page *page = pfn_to_page(pfn);
> +
> +	return is_zone_device_page(page) &&
> +		((page->pgmap->type == MEMORY_DEVICE_FS_DAX) ||
> +		 (page->pgmap->type == MEMORY_DEVICE_DEV_DAX));
> +}

If the mm people agree, I'd prefer something that takes a struct page *
and is exported by include/linux/mm.h.  Then KVM can just do something like

	struct page *page;
	if (!pfn_valid(pfn))
		return true;

	page = pfn_to_page(pfn);
	return PageReserved(page) && !is_dax_page(page);

Thanks,

Paolo