On 29 April 2015 at 10:03, Alexander Graf <agraf@xxxxxxx> wrote: > > > On 18.03.15 20:10, Andrew Jones wrote: >> Introduce a new memory region flag, KVM_MEM_UNCACHED, which >> is needed by ARM. This flag informs KVM that the given memory >> region is typically mapped by the guest as uncached. KVM for >> ARM then maps that region as uncached for userspace as well, >> in order to keep coherency. > > I finally managed to give this a spin and immediately ran into an > unaligned access trap: > > [ 116.509976] Unhandled fault: alignment fault (0x92000061) at > 0x000003ffb1300000 > > Program received signal SIGBUS, Bus error. > [Switching to Thread 0x3ffb317ecb0 (LWP 1956)] > 0x000003ffb685ed68 in memset () from /lib64/libc.so.6 > (gdb) bt > #0 0x000003ffb685ed68 in memset () from /lib64/libc.so.6 > #1 0x000002aaaac013ec in memset (__len=<optimized out>, __ch=0, > __dest=<optimized out>) at /usr/include/bits/string3.h:90 > #2 vbe_ioport_write_data (opaque=0x2aaabbd3600, addr=<optimized out>, > val=65) at /usr/src/debug/qemu-2.3.0-rc4/hw/display/vga.c:739 > #3 0x000002aaaabe07dc in memory_region_write_accessor (mr=<optimized > out>, addr=<optimized out>, value=<optimized out>, size=<optimized out>, > shift=<optimized out>, mask=<optimized out>) at > /usr/src/debug/qemu-2.3.0-rc4/memory.c:430 > [...] This appears to be because the KVM_MEM_UNCACHED flag confusingly isn't marking the memory as Normal-Noncacheable but as Device-nGnRnE (aka Strongly Ordered). You can't unalignedly access Device memory (and so you can't use the usual userspace memcpy, memset, etc). Did we really want Device-nGnRnE memory here? If we did can we have a less confusing name for the flag? [for the non-ARM folk, nGnRnE == no gathering of accesses, no reordering, no early-write-acknowledgement.] thanks -- PMM _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
