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

[Christopher Covington <cov@codeaurora.org>]

On 04/24/2015 08:46 AM, Peter Maydell wrote:
> On 24 April 2015 at 12:26, Paolo Bonzini <pbonzini@redhat.com> 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.

While probably very slow, could the kind of guest memory trace in question be
captured using the GDB stub and a breakpoint on every load an store
instruction (or alternatively, a watchpoint on every possible address)?

Chris

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