Re: [PATCH] KVM: fix async page fault working for readonly mapping

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On 05/22/2012 08:31 AM, Gleb Natapov wrote:
> On Mon, May 21, 2012 at 04:30:41PM -0300, Marcelo Tosatti wrote:
>> On Mon, May 21, 2012 at 04:15:50PM +0800, Xiao Guangrong wrote:
>> > On 05/21/2012 04:08 PM, Gleb Natapov wrote:
>> > 
>> > > On Mon, May 21, 2012 at 02:45:45PM +0800, Xiao Guangrong wrote:
>> > >> If we map a readonly memory space from host to guest and the page is
>> > >> not currently mapped in the host, we will get a fault-pfn and async
>> > >> is not allowed, then the vm will crash
>> > >>
>> > > Why would we want to map a readonly memory space from host to guest?
>> > > We may want to do it to support memory semantics on read and mmio on
>> > > write, but do not right now unless something changed while I was not
>> > > looking.
>> > 
>> > 
>> > Some test cases in kvm-unit-tests and the benchmark i am writing for KVM
>> > need map the function on host to guest.
>> Or ROM. Or read-only mappings of IVSHMEM (which don't exist yet).
> True. KVM should ignore writes to such areas, not kill a guest. Is this
> how the code works today?

Right now qemu maps ROM as RAM.  There is no way to tell kvm that
something is ROM (or ROMD).

There are two options for that:
- mprotect() the ROM, and teach kvm about read-only areas (this patch);
but that doesn't work if we have a read-only and a writable alias of the
same area
- add a flag indicating that an area is ROM or ROMD

I prefer the latter, because of the alias issue.

error compiling committee.c: too many arguments to function
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