Re: [Qemu-devel] [RFC PATCH 0/3] KVM: Introduce KVM_MEM_UNCACHED

Alexander Graf
2015-04-29T08:54:47Z

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
[...]

So maybe forcing the guest to see VRAM as cached is the safer choice
after all?

Alex