On 1/25/19 10:36 PM, Rémi Denis-Courmont wrote: > Hi, > > Le lauantaina 26. tammikuuta 2019, 1.29.27 EET Richard Henderson a écrit : >> On 1/25/19 1:49 PM, Rémi Denis-Courmont wrote: >>> From: Remi Denis-Courmont <remi.denis.courm...@huawei.com> >>> >>> Since QEMU does not support the ARMv8.2-LVA, Large Virtual Address, >>> extension (yet), the VA address space is signed 48-bits. User mode can >>> only handle the positive half of the address space, so that makes a >>> limit of 47 bits. >>> >>> (With LVA, it would be 52 and 51 bits respectively.) >>> >>> The incorrectly large address space conflicts with PAuth instructions, >>> which bits 48-54 and 56-63 for the pointer authentication code. This >>> also conflicts with (as yet unsupported by QEMU) data tagging and with >>> the ARMv8.5-MTE extension. >>> >>> Signed-off-by: Remi Denis-Courmont <remi.denis.courm...@huawei.com> >>> --- >>> >>> target/arm/cpu.h | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/target/arm/cpu.h b/target/arm/cpu.h >>> index ff81db420d..2ccd04b8f7 100644 >>> --- a/target/arm/cpu.h >>> +++ b/target/arm/cpu.h >>> @@ -2503,7 +2503,7 @@ bool write_cpustate_to_list(ARMCPU *cpu); >>> >>> #if defined(TARGET_AARCH64) >>> # define TARGET_PHYS_ADDR_SPACE_BITS 48 >>> >>> -# define TARGET_VIRT_ADDR_SPACE_BITS 64 >>> +# define TARGET_VIRT_ADDR_SPACE_BITS 47 >>> >>> #else >>> # define TARGET_PHYS_ADDR_SPACE_BITS 40 >>> # define TARGET_VIRT_ADDR_SPACE_BITS 32 >> >> This doesn't really conflict, as this doesn't affect much besides the sizing >> of the PageDesc for page_find. > > Uh, that value controls the range of target virtual addresses user mode maps > to host virtual addresses. Without this patch, AFAICT, there are no > guarantees, only chance, that emulated system calls such as mmap() will not > allocate architecturally invalid addresses.
Well, that's the theory, yes. On the plus side, the host platform that virtually everyone uses, x86_64, also uses a 48-bit virtual address space, so there's no chance of such an invalid address. On the minus side, the code within target_mmap is not sufficient to ensure that mmap will not produce invalid addresses. See 76393642ae6, where I extended target/alpha TARGET_VIRT_ADDR_SPACE_BITS to 63 to work around exactly this problem. To wit: target_mmap loops forever asking the kernel for an address below 2**40, which the kernel refused to provide. So changing this value for target/arm runs the risk of causing aarch64-linux-user to fail on a ppc64/s390x/sparc64 host. For all of the tens of people around the world that care about such a configuration. r~
