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(p200300d82f239c00d1f6f7fe8f147e34.dip0.t-ipconnect.de. [2003:d8:2f23:9c00:d1f6:f7fe:8f14:7e34]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45df16b2971sm5934435e9.7.2025.09.09.02.56.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 09 Sep 2025 02:56:50 -0700 (PDT) Message-ID: <57f49b72-2126-48f0-a4ef-4b138bd0bead@redhat.com> Date: Tue, 9 Sep 2025 11:56:47 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 4/7] x86/xen: support nested lazy_mmu sections (again) To: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= , Kevin Brodsky , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Alexander Gordeev , Andreas Larsson , Andrew Morton , Boris Ostrovsky , Borislav Petkov , Catalin Marinas , Christophe Leroy , Dave Hansen , "David S. Miller" , "H. Peter Anvin" , Ingo Molnar , Jann Horn , "Liam R. Howlett" , Lorenzo Stoakes , Madhavan Srinivasan , Michael Ellerman , Michal Hocko , Mike Rapoport , Nicholas Piggin , Peter Zijlstra , Ryan Roberts , Suren Baghdasaryan , Thomas Gleixner , Vlastimil Babka , Will Deacon , Yeoreum Yun , linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, sparclinux@vger.kernel.org, xen-devel@lists.xenproject.org References: <20250908073931.4159362-1-kevin.brodsky@arm.com> <20250908073931.4159362-5-kevin.brodsky@arm.com> <360712fa-f7a0-4674-acc4-76f79141fe4f@suse.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <360712fa-f7a0-4674-acc4-76f79141fe4f@suse.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: YDGiWtlXVrXQdpvcdmERaFFsy8mzsiC78Tm5ZHXly8o_1757411812 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250909_025655_773221_83346612 X-CRM114-Status: GOOD ( 21.13 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 09.09.25 11:37, Jürgen Groß wrote: > On 09.09.25 11:13, David Hildenbrand wrote: >> On 08.09.25 09:39, Kevin Brodsky wrote: >>> Commit 49147beb0ccb ("x86/xen: allow nesting of same lazy mode") >>> originally introduced support for nested lazy sections (LAZY_MMU and >>> LAZY_CPU). It later got reverted by commit c36549ff8d84 as its >>> implementation turned out to be intolerant to preemption. >>> >>> Now that the lazy_mmu API allows enter() to pass through a state to >>> the matching leave() call, we can support nesting again for the >>> LAZY_MMU mode in a preemption-safe manner. If xen_enter_lazy_mmu() is >>> called inside an active lazy_mmu section, xen_lazy_mode will already >>> be set to XEN_LAZY_MMU and we can then return LAZY_MMU_NESTED to >>> instruct the matching xen_leave_lazy_mmu() call to leave >>> xen_lazy_mode unchanged. >>> >>> The only effect of this patch is to ensure that xen_lazy_mode >>> remains set to XEN_LAZY_MMU until the outermost lazy_mmu section >>> ends. xen_leave_lazy_mmu() still calls xen_mc_flush() >>> unconditionally. >>> >>> Signed-off-by: Kevin Brodsky >>> --- >>>   arch/x86/include/asm/paravirt.h       |  6 ++---- >>>   arch/x86/include/asm/paravirt_types.h |  4 ++-- >>>   arch/x86/xen/mmu_pv.c                 | 11 ++++++++--- >>>   3 files changed, 12 insertions(+), 9 deletions(-) >>> >>> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h >>> index 65a0d394fba1..4ecd3a6b1dea 100644 >>> --- a/arch/x86/include/asm/paravirt.h >>> +++ b/arch/x86/include/asm/paravirt.h >>> @@ -529,14 +529,12 @@ static inline void arch_end_context_switch(struct >>> task_struct *next) >>>   #define  __HAVE_ARCH_ENTER_LAZY_MMU_MODE >>>   static inline lazy_mmu_state_t arch_enter_lazy_mmu_mode(void) >>>   { >>> -    PVOP_VCALL0(mmu.lazy_mode.enter); >>> - >>> -    return LAZY_MMU_DEFAULT; >>> +    return PVOP_CALL0(lazy_mmu_state_t, mmu.lazy_mode.enter); >>>   } >>>   static inline void arch_leave_lazy_mmu_mode(lazy_mmu_state_t state) >>>   { >>> -    PVOP_VCALL0(mmu.lazy_mode.leave); >>> +    PVOP_VCALL1(mmu.lazy_mode.leave, state); >>>   } >>>   static inline void arch_flush_lazy_mmu_mode(void) >>> diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/ >>> paravirt_types.h >>> index bc1af86868a3..b7c567ccbf32 100644 >>> --- a/arch/x86/include/asm/paravirt_types.h >>> +++ b/arch/x86/include/asm/paravirt_types.h >>> @@ -45,8 +45,8 @@ typedef int lazy_mmu_state_t; >>>   struct pv_lazy_ops { >>>       /* Set deferred update mode, used for batching operations. */ >>> -    void (*enter)(void); >>> -    void (*leave)(void); >>> +    lazy_mmu_state_t (*enter)(void); >>> +    void (*leave)(lazy_mmu_state_t); >>>       void (*flush)(void); >>>   } __no_randomize_layout; >>>   #endif >>> diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c >>> index 2039d5132ca3..6e5390ff06a5 100644 >>> --- a/arch/x86/xen/mmu_pv.c >>> +++ b/arch/x86/xen/mmu_pv.c >>> @@ -2130,9 +2130,13 @@ static void xen_set_fixmap(unsigned idx, phys_addr_t >>> phys, pgprot_t prot) >>>   #endif >>>   } >>> -static void xen_enter_lazy_mmu(void) >>> +static lazy_mmu_state_t xen_enter_lazy_mmu(void) >>>   { >>> +    if (this_cpu_read(xen_lazy_mode) == XEN_LAZY_MMU) >>> +        return LAZY_MMU_NESTED; >>> + >> >> You mention above "preemption-safe manner" above, so I am wondering, >> what if we get preempted immediately after doing the this_cpu_read() and get >> scheduled on another CPU? >> > > This should still be correct: preemption needs a context switch to happen, > so xen_start_context_switch() and xen_end_context_switch() are involved. > Those are dealing with this problem by doing the right thing in the old > and the new context. Thanks, that makes sense. Would be valuable to add that detail to the patch description. -- Cheers David / dhildenb