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[85.187.216.236]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-42cb7f2e432sm40151444f8f.9.2025.11.26.06.45.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 26 Nov 2025 06:45:51 -0800 (PST) Message-ID: Date: Wed, 26 Nov 2025 16:45:49 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 06/16] x86/virt/tdx: Improve PAMT refcounts allocation for sparse memory To: Rick Edgecombe , bp@alien8.de, chao.gao@intel.com, dave.hansen@intel.com, isaku.yamahata@intel.com, kai.huang@intel.com, kas@kernel.org, kvm@vger.kernel.org, linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org, mingo@redhat.com, pbonzini@redhat.com, seanjc@google.com, tglx@linutronix.de, vannapurve@google.com, x86@kernel.org, yan.y.zhao@intel.com, xiaoyao.li@intel.com, binbin.wu@intel.com Cc: "Kirill A. Shutemov" References: <20251121005125.417831-1-rick.p.edgecombe@intel.com> <20251121005125.417831-7-rick.p.edgecombe@intel.com> From: Nikolay Borisov Content-Language: en-US Autocrypt: addr=nik.borisov@suse.com; keydata= xsFNBGcrpvIBEAD5cAR5+qu30GnmPrK9veWX5RVzzbgtkk9C/EESHy9Yz0+HWgCVRoNyRQsZ 7DW7vE1KhioDLXjDmeu8/0A8u5nFMqv6d1Gt1lb7XzSAYw7uSWXLPEjFBtz9+fBJJLgbYU7G OpTKy6gRr6GaItZze+r04PGWjeyVUuHZuncTO7B2huxcwIk9tFtRX21gVSOOC96HcxSVVA7X N/LLM2EOL7kg4/yDWEhAdLQDChswhmdpHkp5g6ytj9TM8bNlq9I41hl/3cBEeAkxtb/eS5YR 88LBb/2FkcGnhxkGJPNB+4Siku7K8Mk2Y6elnkOctJcDvk29DajYbQnnW4nhfelZuLNupb1O M0912EvzOVI0dIVgR+xtosp66bYTOpX4Xb0fylED9kYGiuEAeoQZaDQ2eICDcHPiaLzh+6cc pkVTB0sXkWHUsPamtPum6/PgWLE9vGI5s+FaqBaqBYDKyvtJfLK4BdZng0Uc3ijycPs3bpbQ bOnK9LD8TYmYaeTenoNILQ7Ut54CCEXkP446skUMKrEo/HabvkykyWqWiIE/UlAYAx9+Ckho TT1d2QsmsAiYYWwjU8igXBecIbC0uRtF/cTfelNGrQwbICUT6kJjcOTpQDaVyIgRSlUMrlNZ XPVEQ6Zq3/aENA8ObhFxE5PLJPizJH6SC89BMKF3zg6SKx0qzQARAQABzSZOaWtvbGF5IEJv cmlzb3YgPG5pay5ib3Jpc292QHN1c2UuY29tPsLBkQQTAQoAOxYhBDuWB8EJLBUZCPjT3SRn XZEnyhfsBQJnK6byAhsDBQsJCAcCAiICBhUKCQgLAgQWAgMBAh4HAheAAAoJECRnXZEnyhfs XbIQAJxuUnelGdXbSbtovBNm+HF3LtT0XnZ0+DoR0DemUGuA1bZAlaOXGr5mvVbTgaoGUQIJ 3Ejx3UBEG7ZSJcfJobB34w1qHEDO0pN9orGIFT9Bic3lqhawD2r85QMcWwjsZH5FhyRx7P2o DTuUClLMO95GuHYQngBF2rHHl8QMJPVKsR18w4IWAhALpEApxa3luyV7pAAqKllfCNt7tmed uKmclf/Sz6qoP75CvEtRbfAOqYgG1Uk9A62C51iAPe35neMre3WGLsdgyMj4/15jPYi+tOUX Tc7AAWgc95LXyPJo8069MOU73htZmgH4OYy+S7f+ArXD7h8lTLT1niff2bCPi6eiAQq6b5CJ Ka4/27IiZo8tm1XjLYmoBmaCovqx5y5Xt2koibIWG3ZGD2I+qRwZ0UohKRH6kKVHGcrmCv0J YO8yIprxgoYmA7gq21BpTqw3D4+8xujn/6LgndLKmGESM1FuY3ymXgj5983eqaxicKpT9iq8 /a1j31tms4azR7+6Dt8H4SagfN6VbJ0luPzobrrNFxUgpjR4ZyQQ++G7oSRdwjfIh1wuCF6/ mDUNcb6/kA0JS9otiC3omfht47yQnvod+MxFk1lTNUu3hePJUwg1vT1te3vO5oln8lkUo9BU knlYpQ7QA2rDEKs+YWqUstr4pDtHzwQ6mo0rqP+zzsFNBGcrpvIBEADGYTFkNVttZkt6e7yA LNkv3Q39zQCt8qe7qkPdlj3CqygVXfw+h7GlcT9fuc4kd7YxFys4/Wd9icj9ZatGMwffONmi LnUotIq2N7+xvc4Xu76wv+QJpiuGEfCDB+VdZOmOzUPlmMkcJc/EDSH4qGogIYRu72uweKEq VfBI43PZIGpGJ7TjS3THX5WVI2YNSmuwqxnQF/iVqDtD2N72ObkBwIf9GnrOgxEyJ/SQq2R0 g7hd6IYk7SOKt1a8ZGCN6hXXKzmM6gHRC8fyWeTqJcK4BKSdX8PzEuYmAJjSfx4w6DoxdK5/ 9sVrNzaVgDHS0ThH/5kNkZ65KNR7K2nk45LT5Crjbg7w5/kKDY6/XiXDx7v/BOR/a+Ryo+lM MffN3XSnAex8cmIhNINl5Z8CAvDLUtItLcbDOv7hdXt6DSyb65CdyY8JwOt6CWno1tdjyDEG 5ANwVPYY878IFkOJLRTJuUd5ltybaSWjKIwjYJfIXuoyzE7OL63856MC/Os8PcLfY7vYY2LB cvKH1qOcs+an86DWX17+dkcKD/YLrpzwvRMur5+kTgVfXcC0TAl39N4YtaCKM/3ugAaVS1Mw MrbyGnGqVMqlCpjnpYREzapSk8XxbO2kYRsZQd8J9ei98OSqgPf8xM7NCULd/xaZLJUydql1 JdSREId2C15jut21aQARAQABwsF2BBgBCgAgFiEEO5YHwQksFRkI+NPdJGddkSfKF+wFAmcr pvICGwwACgkQJGddkSfKF+xuuxAA4F9iQc61wvAOAidktv4Rztn4QKy8TAyGN3M8zYf/A5Zx VcGgX4J4MhRUoPQNrzmVlrrtE2KILHxQZx5eQyPgixPXri42oG5ePEXZoLU5GFRYSPjjTYmP ypyTPN7uoWLfw4TxJqWCGRLsjnkwvyN3R4161Dty4Uhzqp1IkNhl3ifTDYEvbnmHaNvlvvna 7+9jjEBDEFYDMuO/CA8UtoVQXjy5gtOhZZkEsptfwQYc+E9U99yxGofDul7xH41VdXGpIhUj 4wjd3IbgaCiHxxj/M9eM99ybu5asvHyMo3EFPkyWxZsBlUN/riFXGspG4sT0cwOUhG2ZnExv XXhOGKs/y3VGhjZeCDWZ+0ZQHPCL3HUebLxW49wwLxvXU6sLNfYnTJxdqn58Aq4sBXW5Un0Q vfbd9VFV/bKFfvUscYk2UKPi9vgn1hY38IfmsnoS8b0uwDq75IBvup9pYFyNyPf5SutxhFfP JDjakbdjBoYDWVoaPbp5KAQ2VQRiR54lir/inyqGX+dwzPX/F4OHfB5RTiAFLJliCxniKFsM d8eHe88jWjm6/ilx4IlLl9/MdVUGjLpBi18X7ejLz3U2quYD8DBAGzCjy49wJ4Di4qQjblb2 pTXoEyM2L6E604NbDu0VDvHg7EXh1WwmijEu28c/hEB6DwtzslLpBSsJV0s1/jE= In-Reply-To: <20251121005125.417831-7-rick.p.edgecombe@intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 21.11.25 г. 2:51 ч., Rick Edgecombe wrote: > From: "Kirill A. Shutemov" > > init_pamt_metadata() allocates PAMT refcounts for all physical memory up > to max_pfn. It might be suboptimal if the physical memory layout is > discontinuous and has large holes. > > The refcount allocation vmalloc allocation. This is necessary to support a nit: Something's odd with the first sentence, perhaps an "is a" before is missing before "vmalloc"? > large allocation size. The virtually contiguous property also makes it > easy to find a specific 2MB range’s refcount since it can simply be > indexed. > > Since vmalloc mappings support remapping during normal kernel runtime, > switch to an approach that only populates refcount pages for the vmalloc > mapping when there is actually memory for that range. This means any holes > in the physical address space won’t use actual physical memory. > > The validity of this memory optimization is based on a couple assumptions: > 1. Physical holes in the ram layout are commonly large enough for it to be > worth it. > 2. An alternative approach that looks the refcounts via some more layered > data structure wouldn’t overly complicate the lookups. Or at least > more than the complexity of managing the vmalloc mapping. > > Signed-off-by: Kirill A. Shutemov > [Add feedback, update log] > Signed-off-by: Rick Edgecombe > --- > arch/x86/virt/vmx/tdx/tdx.c | 136 +++++++++++++++++++++++++++++++++--- > 1 file changed, 125 insertions(+), 11 deletions(-) > > diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c > index c28d4d11736c..edf9182ed86d 100644 > --- a/arch/x86/virt/vmx/tdx/tdx.c > +++ b/arch/x86/virt/vmx/tdx/tdx.c > @@ -194,30 +194,135 @@ int tdx_cpu_enable(void) > } > EXPORT_SYMBOL_GPL(tdx_cpu_enable); > > -/* > - * Allocate PAMT reference counters for all physical memory. > - * > - * It consumes 2MiB for every 1TiB of physical memory. > - */ > -static int init_pamt_metadata(void) > +/* Find PAMT refcount for a given physical address */ > +static atomic_t *tdx_find_pamt_refcount(unsigned long pfn) > { > - size_t size = DIV_ROUND_UP(max_pfn, PTRS_PER_PTE) * sizeof(*pamt_refcounts); > + /* Find which PMD a PFN is in. */ > + unsigned long index = pfn >> (PMD_SHIFT - PAGE_SHIFT); > > - if (!tdx_supports_dynamic_pamt(&tdx_sysinfo)) > - return 0; > + return &pamt_refcounts[index]; > +} > > - pamt_refcounts = __vmalloc(size, GFP_KERNEL | __GFP_ZERO); > - if (!pamt_refcounts) > +/* Map a page into the PAMT refcount vmalloc region */ > +static int pamt_refcount_populate(pte_t *pte, unsigned long addr, void *data) > +{ > + struct page *page; > + pte_t entry; > + > + page = alloc_page(GFP_KERNEL | __GFP_ZERO); > + if (!page) > return -ENOMEM; > > + entry = mk_pte(page, PAGE_KERNEL); > + > + spin_lock(&init_mm.page_table_lock); > + /* > + * PAMT refcount populations can overlap due to rounding of the > + * start/end pfn. Make sure the PAMT range is only populated once. > + */ > + if (pte_none(ptep_get(pte))) > + set_pte_at(&init_mm, addr, pte, entry); > + else > + __free_page(page); > + spin_unlock(&init_mm.page_table_lock); nit: Wouldn't it be better to perform the pte_none() check before doing the allocation thus avoiding needless allocations? I.e do the alloc/mk_pte only after we are 100% sure we are going to use this entry. > + > return 0; > }