Re: [Qemu-devel] [RFC PATCH v4 02/20] memattrs: add debug attribute

[Brijesh Singh]

Hi Stefan,


On 03/23/2017 06:29 AM, Stefan Hajnoczi wrote:
> On Wed, Mar 08, 2017 at 03:51:28PM -0500, Brijesh Singh wrote:
> > Add a new debug attribute, the attribute should be set when guest memory
> > accesses are performed for debug purposes.
> > The attribute will be used in SEV guest, where we need to distinguish normal
> > vs debug access to guest memory. In debug mode, we need to use SEV commands
> > to access the guest memory.
> >
> > Signed-off-by: Brijesh Singh <address@hidden>
> > ---
> >  include/exec/memattrs.h |    4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/include/exec/memattrs.h b/include/exec/memattrs.h
> > index e601061..b802073 100644
> > --- a/include/exec/memattrs.h
> > +++ b/include/exec/memattrs.h
> > @@ -37,6 +37,8 @@ typedef struct MemTxAttrs {
> >      unsigned int user:1;
> >      /* Requester ID (for MSI for example) */
> >      unsigned int requester_id:16;
> > +    /* Memory access for debug purposes */
>
> What does "debug purposes" mean?  gdbstub?  Can the guest initiate debug
> memory accesses or is the purely QEMU-internal?

What I mean by that is, any access to the guest memory within Qemu internal 
functions
(e.g gdbstub, qemu monitor's memory dump, info mem, info tlb etc). I have also 
introduced
debug version of ldl_phys, ldq_phys, cpu_physical_memory_* [1] and have updated 
hmp monitor
code to use the debug version api's when accessing the guest memory [2].

[1] http://marc.info/?l=qemu-devel&m=148900832814697&w=2
[2] http://marc.info/?l=qemu-devel&m=148900831414693&w=2


Note: SEV debug API's are meant to be used by hypervisor to decrypt/encrypt 
guest memory.
But when you are inside the guest, the  guest will have access to decrypted 
data and does
not need to call down to hypervisor for debug access.


> > +    unsigned int debug:1;
> >  } MemTxAttrs;
> >
> >  /* Bus masters which don't specify any attributes will get this,
> > @@ -46,4 +48,6 @@ typedef struct MemTxAttrs {
> >   */
> >  #define MEMTXATTRS_UNSPECIFIED ((MemTxAttrs) { .unspecified = 1 })
> >
> > +/* Access the guest memory for debug purposes */
> > +#define MEMTXATTRS_DEBUG ((MemTxAttrs) { .debug = 1 })
> >  #endif