Re: [Qemu-devel] About address mapping between host and guest in QEMU

[Peter Maydell]

On 24 April 2015 at 12:26, Paolo Bonzini <address@hidden> wrote:
> On 24/04/2015 04:10, Wenjie Liu wrote:
>> The thing I am trying to achieve is to get the data and guest physical
>> address of every guest memory access, so I need to known which API can
>> be used to do the address transform.
>
> The short answer is that is difficult, because most guest memory
> accesses do not call any C function.  QEMU has a virtual TLB; if you
> have a TLB hit, the code generated by the JIT compiler does the conversion.

In fact on a TLB hit it's possible that it's not actually any
longer determinable what the guest physical address was. This
only really happens with buggy guests, but if the guest does
a load that brings an entry into the TLB and then rewrites the
page table but fails to do the TLB maintenance operation, then
at the point when a subsequent load hits in the TLB we know
the guest virtual address and the host virtual address but not
the guest physical address, and we can't find the guest physaddr
any more even if we walk the page tables, because those have
been changed...

Wanting to do this kind of memory access tracing is a pretty
common user request, though, and it would be nice if QEMU
had the facilities for it. It's just that our current design
is really not set up to make it easy to implement.

-- PMM