On 04/19/2012 11:59 AM, Gleb Natapov wrote: > On Wed, Apr 18, 2012 at 07:22:47PM +0300, Avi Kivity wrote: > > MMIO that are split across a page boundary are currently broken - the > > code does not expect to be aborted by the exit to userspace for the > > first MMIO fragment. > > > > This patch fixes the problem by generalizing the current code for handling > > 16-byte MMIOs to handle a number of "fragments", and changes the MMIO > > code to create those fragments. > > > For 16 bit IO userspace will see two 8bit writes. Is this OK? I believe so. The Pentium bus was 64-bit wide with no address lines A0-A2, instead it had byte enables BE0-BE7. So a write that crosses a quadword boundary would be split. Similarly the 32-bit PCI bus has 4 byte enables, so anything that crosses a dword boundary is split. LOCKed ops are implemented by asserting a signal during the two operations. No idea how it's implemented with modern busless processors, but the same semantics have been kept, probably. Even more interesting is that a 32-bit write is seen as an 8 bit write followed by a 24-bit write. Note also that the two accesses can target different devices. > Is there > real code that does this kind of IO, or the patch is for correctness > only? I should have mentioned it in the changelog - it's Windows 95. There is also a Red Hat Linux version that does something like this, I remember adding some code in qemu-kvm to handle the 3-byte write. So I guess this fixes a regression. (not sure - in the Windows 95 case we have a write to VGA that crosses a page boundary (but the pages are physically contiguous, nothing strange is going on; the RHL case was writing partially to RAM and partially to unallocated memory, so it might work with the current code). -- error compiling committee.c: too many arguments to function -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
