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AJvYcCXQMBKf7k/93Rvc6pm/Kv4UpNxcq/x4EGlNgYFyvjAQRwgNO+9GvlRI6/0L8u9JORaFAO6Jlz4yB/O+vRs2@vger.kernel.org X-Gm-Message-State: AOJu0YwSPcY2lsw4dF7utB6mxfIKXNZPwPRDmVJX3JiwjzNOx/zQpQ4N ot7B2yJnqhPvko8ZphoOiqelj2tszmDwRBFnp9ngrGugh3Zv9ca9CmYvomd1EPGVCVTQJ8VlQjs xg5PQwA== X-Google-Smtp-Source: AGHT+IG6LzfASe3puScn/Qv3ruREwfR064s8V+HL6PEjTR9Oas02f/m6vRYzgkbGrD2ayUhiXQ3k4vsZIjY= X-Received: from pfbli4.prod.google.com ([2002:a05:6a00:7184:b0:746:301b:10ca]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a20:431f:b0:21a:da01:e0cf with SMTP id adf61e73a8af0-21ee261054cmr1284527637.22.1749161530718; Thu, 05 Jun 2025 15:12:10 -0700 (PDT) Date: Thu, 5 Jun 2025 15:12:09 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: linux-arm-msm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250605153800.557144-1-tabba@google.com> <20250605153800.557144-19-tabba@google.com> Message-ID: Subject: Re: [PATCH v11 18/18] KVM: selftests: guest_memfd mmap() test when mapping is allowed From: Sean Christopherson To: James Houghton 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 On Thu, Jun 05, 2025, James Houghton wrote: > On Thu, Jun 5, 2025 at 8:38=E2=80=AFAM Fuad Tabba wrot= e: > > @@ -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 total= _size) > > +{ > > + const char val =3D 0xaa; > > + char *mem; >=20 > This must be `volatile char *` to ensure that the compiler doesn't > elide the accesses you have written. >=20 > > + size_t i; > > + int ret; > > + > > + mem =3D mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_PRIV= ATE, fd, 0); > > + TEST_ASSERT(mem =3D=3D MAP_FAILED, "Copy-on-write not allowed b= y guest_memfd."); > > + > > + mem =3D mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_SHAR= ED, fd, 0); > > + TEST_ASSERT(mem !=3D MAP_FAILED, "mmap() for shared guest memor= y should succeed."); > > + > > + memset(mem, val, total_size); >=20 > Now unfortunately, `memset` and `munmap` will complain about the > volatile qualification. So... >=20 > memset((char *)mem, val, total_size); >=20 > Eh... wish they just wouldn't complain, but this is a small price to > pay for correctness. :) >=20 > > + for (i =3D 0; i < total_size; i++) > > + TEST_ASSERT_EQ(mem[i], val); >=20 > 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. > [1]: https://godbolt.org/z/Wora54bP6 >=20 > Feel free to add `volatile` to that snippet to see how the code changes.