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[69.159.169.238]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-8ac84c45944sm6173456d6.24.2026.04.09.14.24.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 09 Apr 2026 14:24:40 -0700 (PDT) Message-ID: <4f8f3e44-7d5b-4b8a-baeb-f42fb3e8a074@redhat.com> Date: Thu, 9 Apr 2026 17:24:39 -0400 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 v3 10/10] mm: thp: x86: cleanup PSE feature bit usage To: Dave Hansen , Zi Yan Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, david@kernel.org, baolin.wang@linux.alibaba.com, ryan.roberts@arm.com, akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, Dave Hansen , Thomas Gleixner , Ingo Molnar , Borislav Petkov References: <16fe2db0becab98db70f53e4ce1422450a9f40ee.1775679721.git.luizcap@redhat.com> <631bceef-d32b-475f-9d1a-644593aceddc@intel.com> Content-Language: en-US, en-CA From: Luiz Capitulino In-Reply-To: <631bceef-d32b-475f-9d1a-644593aceddc@intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2026-04-09 16:10, Dave Hansen wrote: > On 4/9/26 08:57, Zi Yan wrote: >> Historically, THP support on x86 checked the PSE feature bit to enable >> THP. On 64-bit, this check is redundant since PSE is always enabled by >> default for compatibility. > > What do you mean by "PSE is always enabled by default"? > > A hypervisor is completely free to not enumerate PSE support to a guest. > Any kernel can disable PSE with clearcpuid=3 on the command-line. PSE is > also disabled for at least one erratum (which I think includes some > 64-bit CPUs). You're right, I was wrong to assume that PSE could be ignored on 64-bit. Thanks for taking the time to show it. I'll drop this patch for v4. > > Here's a snippet in a 64-bit VM of /proc/cpuinfo on a normal boot: > > DirectMap4k: 83836 kB > DirectMap2M: 1939456 kB > DirectMap1G: 1048576 kB > > and with clearcpuid=3: > > DirectMap4k: 3071868 kB > DirectMap2M: 0 kB > > Just to show that it's possible. > > So perhaps I'm reading into the changelog too much, but this doesn't > quite seem right to me. >