Re: [PATCH v5 01/23] memattrs: add debug attribute

From: "Edgar E. Iglesias" <edgar.iglesias@xilinx.com>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: Brijesh Singh <brijesh.singh@amd.com>,
	QEMU Developers <qemu-devel@nongnu.org>,
	Alistair Francis <alistair.francis@xilinx.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Cornelia Huck <cornelia.huck@de.ibm.com>,
	"Daniel P . Berrange" <berrange@redhat.com>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Eduardo Habkost <ehabkost@redhat.com>,
	Eric Blake <eblake@redhat.com>, kvm-devel <kvm@vger.kernel.org>,
	Marcel Apfelbaum <marcel@redhat.com>,
	Markus Armbruster <armbru@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Peter Crosthwaite <crosthwaite.peter@gmail.com>,
	Richard Henderson <richard.henderson@linaro.org>,
	Richard Henderson <rth@twiddle.net>,
	Stefan Hajnoczi <stefanha@gmail.com>,
	Thomas Len
Subject: Re: [PATCH v5 01/23] memattrs: add debug attribute
Date: Fri, 8 Dec 2017 17:24:22 +0700	[thread overview]
Message-ID: <20171208102422.GA26889@toto> (raw)
In-Reply-To: <CAFEAcA_A9c3QMOJeHK+gyTpwkxo0hpkU9jhCdvQnpfbGBw+Wfg@mail.gmail.com>

On Fri, Dec 08, 2017 at 09:55:26AM +0000, Peter Maydell wrote:
> On 7 December 2017 at 21:20, Brijesh Singh <brijesh.singh@amd.com> wrote:
> > On 12/06/2017 04:03 PM, Peter Maydell wrote:
> >> For instance, if a device gets a debug=1 transaction
> >> should it refuse to do things like read-clears-bits
> >> semantics or other side-effects you wouldn't expect
> >> of debugger accesses?
> >>
> >
> > Sorry I am not able to understand "if a device gets a debug=1 transition"
> > comment, Can you please explain me a bit more.
> 
> A device model (eg a UART) can be written to look at the
> MemTxAttrs that it's passed and behave differently if debug=1.
> 
> > If you give me example on how
> > to trigger this type of request with debug=1 then I can look into the code
> > and see what we can do when memory encryption is enabled. The things like
> > read-clears-bits semantics will be tricky.
> 
> The question was really whether we want to make this a general
> indicator of "this operation was triggered by a debugger" and
> expand that to mean "don't do things that mess with the state
> of the simulation unexpectedly", or if this is really a very
> encrypted-memory specific thing.

IMO, It sounds like a good idea to make it a general indicator of
debugger access. We may not need to be very strict about an
exact global meaning, but individual device models may for example
choose to expose additional debug registers and features or
to avoid certain register access side-effects.

In some cases it may be confusing to change the behaviour of
existing regs with side-effects based on debug access, so it may
not always make sense.

Cheers,
Edgar

> 
> By the way, I don't think this:
> 
> > /* Access the guest memory for debug purposes */
> > #define MEMTXATTRS_DEBUG ((MemTxAttrs) { .debug = 1 })
> 
> is a great idea. Callers that care about the transaction
> attributes should just specify them properly. MEMTXATTRS_UNSPECIFIED
> is a fallback for the large set of places that don't care at all.
> 
> thanks
> -- PMM