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AJvYcCV5sObJafuNNaaCgzQ6ffVUnx0ZqaN3wQCUlfQU7s/noadcDfZaAxxPCcDXaT8HD696cms90ykrKQ==@kvack.org X-Gm-Message-State: AOJu0YzpxGCDhTGk82oJU1qXr7xtd6gf0lmqZmaTc+ZD3c8jxWURxxMy NJ/57SFFtSOkGyN/vz7riPs8WK9IRCamjqqIkNpxlRHd7v6I+Lwsf1rjbJZhJJzpUQnBOIWZF6/ 9+0FlYkA5NlFB9Ac+Yu0FaWODiJI55YbC4n4fDluT X-Gm-Gg: ASbGnctFeFpwbWfQLcb5ZKhHX92FqC3/GbOB/URM3KzcXw+M5IhYsaJWpJtjwesdYO/ 9uz9Gj5StkI3XQvFSIPj+ZJRNpJiudO9AaYfF34KxnZ8TRN8jKjAhqYEOiz+MPJ3SPc0cRs8bCF 79BD1E0REPXwNEvgf/F+NeMKcDmoMeOTW7/GeJlZlsLbu1FV0ztVrTe9U6RJ3tD8AqGoIot/Cqt dOGDXb62mA07NSQpwPpxZDCcz9j9AdPK5M76ZhQBwzheuHPceQRANaG1iUzixYcMypvbi6RMn08 Pj2Q4lt+XJ+/p050UtXWXW6mEPD/ X-Google-Smtp-Source: AGHT+IGVKWbjRmm9tgU6uLDNVHu5xkhvGzmzSejdcqRBFvmWHeadfJ39slfnN2SFI/bj9haEVWPhElX75HTn7j06Rtw= X-Received: by 2002:a05:7022:4a:b0:11a:fcc8:c311 with SMTP id a92af1059eb24-11dc9c5d1ffmr422435c88.5.1764553670544; Sun, 30 Nov 2025 17:47:50 -0800 (PST) MIME-Version: 1.0 References: <20251113230759.1562024-1-michael.roth@amd.com> <20251113230759.1562024-4-michael.roth@amd.com> <20251121130144.u7eeaafonhcqf2bd@amd.com> In-Reply-To: From: Vishal Annapurve Date: Sun, 30 Nov 2025 17:47:37 -0800 X-Gm-Features: AWmQ_bndv783uhaZZ_oV6Xm0iV6cY4XqsD5JvxFNZOjPYdlSyliY7wOlOHpUWNk Message-ID: Subject: Re: [PATCH 3/3] KVM: guest_memfd: GUP source pages prior to populating guest memory To: Yan Zhao Cc: Michael Roth , kvm@vger.kernel.org, linux-coco@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, thomas.lendacky@amd.com, pbonzini@redhat.com, seanjc@google.com, vbabka@suse.cz, ashish.kalra@amd.com, liam.merwick@oracle.com, david@redhat.com, ackerleytng@google.com, aik@amd.com, ira.weiny@intel.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 21A1340014 X-Stat-Signature: sind6ptzsuwmqotp81qorpo1xzj4iqzo X-HE-Tag: 1764553671-635114 X-HE-Meta: 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 yIwAbfHV aVdPIi2zNQwx8DtVKby6OUaOj+OdBHhjfbgicRmIueCgfVDM7g5XYB0bsuAeYw8ZKaAblX0pU5z64fa0EV/it79ygZVlfM8epbkExe/mpnGMEp4BUpKVuuqnF56caw4vfIfKZpd85iG6kWm5bVAYGFaTeDWNkIbfCXpkblvJ9Nj2qDocwnKqdHIaNp1d1+BMqp9iiBADVewOVFKWzTT2ylmsbnIlJWm/3Ug9+ X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Nov 24, 2025 at 1:34=E2=80=AFAM Yan Zhao wro= te: > > > > > + if (src_offset) { > > > > + src_vaddr =3D kmap_local_pfn(page_to_pfn(= src_pages[i + 1])); > > > > + > > > > + memcpy(dst_vaddr + PAGE_SIZE - src_offset= , src_vaddr, src_offset); > > > > + kunmap_local(src_vaddr); > > > IIUC, src_offset is the src's offset from the first page. e.g., > > > src could be 0x7fea82684100, with src_offset=3D0x100, while npages co= uld be 512. > > > > > > Then it looks like the two memcpy() calls here only work when npages = =3D=3D 1 ? > > > > src_offset ends up being the offset into the pair of src pages that we > > are using to fully populate a single dest page with each iteration. So > > if we start at src_offset, read a page worth of data, then we are now a= t > > src_offset in the next src page and the loop continues that way even if > > npages > 1. > > > > If src_offset is 0 we never have to bother with straddling 2 src pages = so > > the 2nd memcpy is skipped on every iteration. > > > > That's the intent at least. Is there a flaw in the code/reasoning that = I > > missed? > Oh, I got you. SNP expects a single src_offset applies for each src page. > > So if npages =3D 2, there're 4 memcpy() calls. > > src: |---------|---------|---------| (VA contiguous) > ^ ^ ^ > | | | > dst: |---------|---------| (PA contiguous) > > > I previously incorrectly thought kvm_gmem_populate() should pass in src_o= ffset > as 0 for the 2nd src page. > > Would you consider checking if params.uaddr is PAGE_ALIGNED() in > snp_launch_update() to simplify the design? > IIUC, this ship has sailed, as asserting this would break existing userspace which can pass unaligned userspace buffers.