On Thu, Aug 16, 2012 at 12:03:01PM +0300, Avi Kivity wrote: > On 08/15/2012 08:53 PM, Marcelo Tosatti wrote: > > On Wed, Aug 15, 2012 at 01:44:14PM +0300, Avi Kivity wrote: > >> On 08/14/2012 06:51 PM, Marcelo Tosatti wrote: > >> >> > >> >> Userspace may want to modify the ROM (for example, when programming a > >> >> flash device). It is also possible to map an hva range rw through one > >> >> slot and ro through another. > >> > > >> > Right, can do that with multiple userspace maps to the same anonymous > >> > memory region (see other email). > >> > >> Yes it's possible. It requires that we move all memory allocation to be > >> fd based, since userspace can't predict what memory will be dual-mapped > >> (at least if emulated hardware allows this). > > > > It can: > > - Create named memory object, with associated fd. > > - Copy data from large anonymous memory region to named memory. > > That doesn't work if dma is in progress (assigned device). It also > doubles the amount of memory in use. > > > - Unmap region that must be dual-mapped from large anonymous memory chunk. > > - Map named memory object at address. > > > > The last step can be replaced by adjusting KVM memory slots. > > > > The disadvantage of protection information in memory slots > > is that it duplicates functionality that is handled by > > userspace mappings. > > Agree. So does the memory slots mechanism, and even dirty logging. > > > Moreover, multiple memory maps are necessary for any > > split-qemu-into-smaller-pieces solutions. > > Complex users can use complex mechanism, but let's keep the simple stuff > simple. > > > > >> Is this a reasonable > >> requirement? Do ksm/thp/autonuma work with this? > > > > As mentioned, only memory used for ROM purposes must be dual mapped. > > > > I don't think there is any way to create multiple mappings > > to one anonymous memory object ATM, but POSIX defines it > > (posix_typed_mem_open). > > > > The limitation of thp/ksm on shared memory also affects any other user > > of shared memory, so it should be fixed there. > > > > Also, QEMU ROM is allocated separately from RAM, correct? > > > > Correct. But the chipset is also able to to write-protect some ranges > in the 0xc0000-0x100000 area via the PAM. It is able to write-protect > both RAM and PCI memory (usually mapped to flash). You are convinced that adding read-write protection information to the memory slots, which controls access by the guest, in addition to the userspace host pagetables, is useful. OK. -- 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
