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AJvYcCW/EpwWSQ9C5rstZGsnCX/2Ft5XiSeOUEVTnrlvxaJ/l/W+024lq27es9QMIOKyyyjLFDktAoa9/Q==@kvack.org X-Gm-Message-State: AOJu0YzqnOBCLhkyZZ5b9rcU5mSLkEbWpvMv5gtVTe7vZWH3LnT5NuzL cg5IRKaun66IP5uM0nzMovRHri81hCBAkA/YqB572nAKyazNX6tlljeXSMHRfLEEMQTdV5RqYvE Dp1LGTgSP3atZH+FWGpa6ZvbecyS57n2j6aXifZIq X-Gm-Gg: ASbGnctWlyFa/U8luAUyOR6SBN50ypT+X9PomgGPsHQvA4nNhyk1Hyq7TjQb+w+yhoA /nGafZ2TKWfhkfRZB0AgKLRj90CDwbG/aLu1wsegwbSz1eeN1X7D5hlKdYcE+5Ag/4CeaZ2paBT kVMwQ8OT96wEvu3julSYJQyfCNiUcw5owqXW8GbsyX1DVaQF0YHoBa9qqxpbvnXdNbzZ0C7d9z X-Google-Smtp-Source: AGHT+IEp6IGPuzz6J/+BpJ6aDeMRiVbGyP9uhXLN3ZH8ojjYUdl2R1SNFOG6TloGaR9kARIO3zOdCXd1Ty9ye/gS72k= X-Received: by 2002:a05:6902:1881:b0:e81:5197:3dd5 with SMTP id 3f1490d57ef6-e81a2276cfdmr2085466276.1.1749161882078; Thu, 05 Jun 2025 15:18:02 -0700 (PDT) MIME-Version: 1.0 References: <20250605153800.557144-1-tabba@google.com> <20250605153800.557144-19-tabba@google.com> In-Reply-To: From: James Houghton Date: Thu, 5 Jun 2025 15:17:26 -0700 X-Gm-Features: AX0GCFusmx5LnqBezumi2CBPX3jBsMOV_ncGvRUPGLV-0hofjeATZJ_DcmP1Prc Message-ID: Subject: Re: [PATCH v11 18/18] KVM: selftests: guest_memfd mmap() test when mapping is allowed To: Sean Christopherson Cc: Fuad Tabba , kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, kvmarm@lists.linux.dev, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, viro@zeniv.linux.org.uk, brauner@kernel.org, willy@infradead.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, vannapurve@google.com, ackerleytng@google.com, mail@maciej.szmigiero.name, david@redhat.com, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_eberman@quicinc.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, qperret@google.com, keirf@google.com, roypat@amazon.co.uk, shuah@kernel.org, hch@infradead.org, jgg@nvidia.com, rientjes@google.com, jhubbard@nvidia.com, fvdl@google.com, hughd@google.com, peterx@redhat.com, pankaj.gupta@amd.com, ira.weiny@intel.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 9519B14000B X-Stat-Signature: 1o5fa3tuftc9dqwroafcxnnckin7hukh X-Rspam-User: X-HE-Tag: 1749161883-972528 X-HE-Meta: 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 /aAmzUM3 TkRbDiWC6hZuPEFHiYSPJtlSq+2aBXuX2PuwxfydNhbWjFG1BNwBE5+DFqa5WnZvmQg6OUsqnAiUCmEP1nXKMtEJ2SEWrjtLC4SnZXapJIsKui9C+lerxAZNkXVRZGX98n96zpqsvm9SatfUpwy1yZLq7sEm8NXKNBmQ+JL5mzptCmQTTX6TAwuNN42pZWf7At2Dp/Aaqzl4CIkS/qwxj3vfK6btpQ0JI7sDAhjE9ZxLEubstIzoUuEGVDJdAol4G1jf9+cGSVQOM8BIXBmjj5xXEmTg6uh2COswks9k4ZsBrw1qBVeAgnJPk/wd+KxM97SmwhWkp1OdjBXRcIxNiyeZVRsqIF6jN3HltpXdoJPWfVFvdcDGIHcSFxjv6RLSfAj9T0kXzxHaUfiQ= 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 Thu, Jun 5, 2025 at 3:12=E2=80=AFPM Sean Christopherson wrote: > > On Thu, Jun 05, 2025, James Houghton wrote: > > On Thu, Jun 5, 2025 at 8:38=E2=80=AFAM Fuad Tabba wr= ote: > > > @@ -34,12 +36,83 @@ static void test_file_read_write(int fd) > > > "pwrite on a guest_mem fd should fail"); > > > } > > > > > > -static void test_mmap(int fd, size_t page_size) > > > +static void test_mmap_supported(int fd, size_t page_size, size_t tot= al_size) > > > +{ > > > + const char val =3D 0xaa; > > > + char *mem; > > > > This must be `volatile char *` to ensure that the compiler doesn't > > elide the accesses you have written. > > > > > + size_t i; > > > + int ret; > > > + > > > + mem =3D mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_PR= IVATE, fd, 0); > > > + TEST_ASSERT(mem =3D=3D MAP_FAILED, "Copy-on-write not allowed= by guest_memfd."); > > > + > > > + mem =3D mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_SH= ARED, fd, 0); > > > + TEST_ASSERT(mem !=3D MAP_FAILED, "mmap() for shared guest mem= ory should succeed."); > > > + > > > + memset(mem, val, total_size); > > > > Now unfortunately, `memset` and `munmap` will complain about the > > volatile qualification. So... > > > > memset((char *)mem, val, total_size); > > > > Eh... wish they just wouldn't complain, but this is a small price to > > pay for correctness. :) > > > > > + for (i =3D 0; i < total_size; i++) > > > + TEST_ASSERT_EQ(mem[i], val); > > > > The compiler is allowed to[1] elide the read of `mem[i]` and just > > assume that it is `val`. > > I don't think "volatile" is needed. Won't READ_ONCE(mem[i]) do the trick= ? That > in turn will force the compiler to emit the stores as well. Yeah `volatile` is only needed on the reads. READ_ONCE() implies a `volatile` read, so if you want to write it that way, that's fine too. I prefer my original suggestion though; it's less likely for there to be a bug. :) > > [1]: https://godbolt.org/z/Wora54bP6 > > > > Feel free to add `volatile` to that snippet to see how the code changes= .