Re: [PATCH v3] memory: Directly dispatch alias accesses on origin memory region

From: Peter Maydell <peter.maydell@linaro.org>
To: Peter Xu <peterx@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	"Alexey Kardashevskiy" <aik@ozlabs.ru>,
	"Richard Henderson" <richard.henderson@linaro.org>,
	"Mark Cave-Ayland" <mark.cave-ayland@ilande.co.uk>,
	"Philippe Mathieu-Daudé" <f4bug@amsat.org>,
	"QEMU Developers" <qemu-devel@nongnu.org>,
	"Jan Kiszka" <jan.kiszka@web.de>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Igor Mammedov" <imammedo@redhat.com>,
	"David Gibson" <david@gibson.dropbear.id.au>
Subject: Re: [PATCH v3] memory: Directly dispatch alias accesses on origin memory region
Date: Wed, 21 Apr 2021 12:05:08 +0100	[thread overview]
Message-ID: <CAFEAcA9Q+tSb6T5N-mkZjS1CZr05AErfPPcWccYGGw9hcHKuXA@mail.gmail.com> (raw)
In-Reply-To: <20210420205933.GF4440@xz-x1>

On Tue, 20 Apr 2021 at 21:59, Peter Xu <peterx@redhat.com> wrote:
> I think it should always be a valid request to trigger memory access via the MR
> layer, say, what if the caller has no address space context at all? From the
> name of memory_region_dispatch_write|read I don't see either on why we should
> not take care of alias mrs.  That's also the reason I'd even prefer this patch
> rather than an assert.

It's a bit of an odd case to need to do direct accesses on an MR
(so if you think you need to you should probably look for whether there's
a better way to do it), but there are sometimes reasons it's necessary or
expedient -- we have about half a dozen uses which aren't part of the core
memory system using memory_region_dispatch_* as an internal function.
So I think I agree that while we have this as an API exposed to
the rest of the system it would be nice if it Just Worked for
alias MRs.

thanks
-- PMM