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[79.242.62.5]) by smtp.gmail.com with ESMTPSA id l35sm192133wms.40.2021.09.03.12.37.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 03 Sep 2021 12:37:40 -0700 (PDT) Subject: Re: [PATCH v4 7/9] migration: Simplify alignment and alignment checks To: Peter Xu References: <20210902131432.23103-1-david@redhat.com> <20210902131432.23103-8-david@redhat.com> <8985fca4-e15c-95a7-2c45-74353a3a19fa@redhat.com> <2d04741d-94bd-9cd2-4d70-f6505bc8db16@redhat.com> From: David Hildenbrand Organization: Red Hat Message-ID: <5da476a5-cf79-c9b5-1116-a96f5349680e@redhat.com> Date: Fri, 3 Sep 2021 21:37:38 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=david@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.392, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.888, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Eduardo Habkost , Juan Quintela , Pankaj Gupta , "Michael S. Tsirkin" , teawater , qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , Alex Williamson , Marek Kedzierski , Paolo Bonzini , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , Andrey Gruzdev , Wei Yang Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 03.09.21 21:14, Peter Xu wrote: > On Fri, Sep 03, 2021 at 12:07:20PM +0200, David Hildenbrand wrote: >> On 03.09.21 10:47, David Hildenbrand wrote: >>> On 03.09.21 00:32, Peter Xu wrote: >>>> On Thu, Sep 02, 2021 at 03:14:30PM +0200, David Hildenbrand wrote: >>>>> diff --git a/migration/migration.c b/migration/migration.c >>>>> index bb909781b7..ae97c2c461 100644 >>>>> --- a/migration/migration.c >>>>> +++ b/migration/migration.c >>>>> @@ -391,7 +391,7 @@ int migrate_send_rp_message_req_pages(MigrationIncomingState *mis, >>>>> int migrate_send_rp_req_pages(MigrationIncomingState *mis, >>>>> RAMBlock *rb, ram_addr_t start, uint64_t haddr) >>>>> { >>>>> - void *aligned = (void *)(uintptr_t)(haddr & (-qemu_ram_pagesize(rb))); >>>>> + void *aligned = (void *)QEMU_ALIGN_DOWN(haddr, qemu_ram_pagesize(rb)); >>>> >>>> Is uintptr_t still needed? I thought it would generate a warning otherwise but >>>> not sure. >>> >>> It doesn't in my setup, but maybe it will on 32bit archs ... >>> >>> I discussed this with Phil in >>> >>> https://lkml.kernel.org/r/2c8d80ad-f171-7d5f-3235-92f02fa174b3@redhat.com >>> >>> Maybe >>> >>> QEMU_ALIGN_PTR_DOWN((void *)haddr, qemu_ram_pagesize(rb))); >>> >>> Is really what we want. >> >> ... but it would suffer the same issue I think. I just ran it trough the >> gitlab pipeline, including "i386-fedora-cross-compile" ... and it seems to >> compile just fine, which is weird, because I'd also expect >> >> "warning: cast to pointer from integer of different size >> [-Wint-to-pointer-cast]" >> >> We most certainly need the "(void *)(uintptr_t)" to convert from u64 to a >> pointer. >> >> Let's just do it cleanly: >> >> void *unaligned = (void *)(uintptr_t)haddr; >> void *aligned = QEMU_ALIGN_PTR_DOWN(unaligned, qemu_ram_pagesize(rb)); >> >> Thoughts? > > ---8<--- > $ cat a.c > #include > #include > #include > > #define ROUND_DOWN(n, d) ((n) & -(0 ? (n) : (d))) > #define QEMU_ALIGN_DOWN(n, m) ((n) / (m) * (m)) > > unsigned long getns(void) > { > struct timespec tp; > > clock_gettime(CLOCK_MONOTONIC, &tp); > return tp.tv_sec * 1000000000 + tp.tv_nsec; > } > > void main(void) > { > int i; > unsigned long start, end, v1 = 0x1234567890, v2 = 0x1000; > > start = getns(); > for (i = 0; i < 1000000; i++) { > v1 = ROUND_DOWN(v1, v2); > } > end = getns(); > printf("ROUND_DOWN took: \t%ld (us)\n", (end - start) / 1000); > > start = getns(); > for (i = 0; i < 1000000; i++) { > v1 = QEMU_ALIGN_DOWN(v1, v2); > } > end = getns(); > printf("QEMU_ALIGN_DOWN took: \t%ld (us)\n", (end - start) / 1000); > } > $ make a > $ ./a > ROUND_DOWN took: 1445 (us) > QEMU_ALIGN_DOWN took: 9684 (us) > ---8<--- > > So it's ~5 times slower here on the laptop, even if not very stable. Agree > it's not a big deal. :) Same results for me, especially even if I turn v1 and v2 into global volatiles, make sure the results won't get optimized out and compile with -03. > > It's just that since we know it's still faster, I then second: > > (uinptr_t)ROUND_DOWN(...); Well okay then, void *aligned = (void *)(uintptr_t)ROUND_DOWN(haddr, qemu_ram_pagesize(rb)); fits precisely into a single line :) -- Thanks, David / dhildenb