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Date:   Tue, 31 Oct 2023 16:35:08 +0800
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Subject: Re: [PATCH v13 17/35] KVM: Add transparent hugepage support for
 dedicated guest memory
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To:     Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        Huacai Chen <chenhuacai@kernel.org>,
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        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
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        Xu Yilun <yilun.xu@intel.com>,
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        Fuad Tabba <tabba@google.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
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        David Matlack <dmatlack@google.com>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        Isaku Yamahata <isaku.yamahata@intel.com>,
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        Maciej Szmigiero <mail@maciej.szmigiero.name>,
        David Hildenbrand <david@redhat.com>,
        Quentin Perret <qperret@google.com>,
        Michael Roth <michael.roth@amd.com>,
        Wang <wei.w.wang@intel.com>,
        Liam Merwick <liam.merwick@oracle.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
References: <20231027182217.3615211-1-seanjc@google.com>
 <20231027182217.3615211-18-seanjc@google.com>
From:   Xiaoyao Li <xiaoyao.li@intel.com>
In-Reply-To: <20231027182217.3615211-18-seanjc@google.com>
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On 10/28/2023 2:21 AM, Sean Christopherson wrote:
> Extended guest_memfd to allow backing guest memory with transparent 
> hugepages. Require userspace to opt-in via a flag even though there's no 
> known/anticipated use case for forcing small pages as THP is optional, 
> i.e. to avoid ending up in a situation where userspace is unaware that 
> KVM can't provide hugepages.

Personally, it seems not so "transparent" if requiring userspace to opt-in.

People need to 1) check if the kernel built with TRANSPARENT_HUGEPAGE 
support, or check is the sysfs of transparent hugepage exists; 2)get the 
maximum support hugepage size 3) ensure the size satisfies the 
alignment; before opt-in it.

Even simpler, userspace can blindly try to create guest memfd with 
transparent hugapage flag. If getting error, fallback to create without 
the transparent hugepage flag.

However, it doesn't look transparent to me.