Re: [PATCH v5 00/12] KVM: introduce readonly memslot

[Avi Kivity <avi@redhat.com>]

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).



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