linux-kernel - RE: [RFC PATCH v11 12/29] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

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Date:   Tue, 25 Jul 2023 15:09:29 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        "Michael Ellerman" <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>,
        "Paul Walmsley" <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "kvm-riscv@...ts.infradead.org" <kvm-riscv@...ts.infradead.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        "Yu Zhang" <yu.c.zhang@...ux.intel.com>,
        "Annapurve, Vishal" <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        "Maciej Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Liam Merwick <liam.merwick@...cle.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: RE: [RFC PATCH v11 12/29] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for
 guest-specific backing memory

On Wednesday, July 19, 2023 7:45 AM, Sean Christopherson wrote:
> +int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> +		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order) {
> +	pgoff_t index = gfn - slot->base_gfn + slot->gmem.pgoff;
> +	struct kvm_gmem *gmem;
> +	struct folio *folio;
> +	struct page *page;
> +	struct file *file;
> +
> +	file = kvm_gmem_get_file(slot);
> +	if (!file)
> +		return -EFAULT;
> +
> +	gmem = file->private_data;
> +
> +	if (WARN_ON_ONCE(xa_load(&gmem->bindings, index) != slot)) {
> +		fput(file);
> +		return -EIO;
> +	}
> +
> +	folio = kvm_gmem_get_folio(file_inode(file), index);
> +	if (!folio) {
> +		fput(file);
> +		return -ENOMEM;
> +	}
> +
> +	page = folio_file_page(folio, index);
> +
> +	*pfn = page_to_pfn(page);
> +	*max_order = compound_order(compound_head(page));

Maybe better to check if caller provided a buffer to get the max_order:
if (max_order)
	*max_order = compound_order(compound_head(page));

This is what the previous version did (restrictedmem_get_page),
so that callers who only want to get a pfn don't need to define
an unused "order" param.