Re: [Qemu-devel] release retrospective, next release timing, numbering

From: Laszlo Ersek <lersek@redhat.com>
To: "Richard Henderson" <richard.henderson@linaro.org>,
	"Michal Suchánek" <msuchanek@suse.de>,
	"Peter Maydell" <peter.maydell@linaro.org>
Cc: Thomas Huth <thuth@redhat.com>,
	QEMU Developers <qemu-devel@nongnu.org>,
	Stefan Hajnoczi <stefanha@redhat.com>
Subject: Re: [Qemu-devel] release retrospective, next release timing, numbering
Date: Wed, 2 May 2018 12:41:21 +0200	[thread overview]
Message-ID: <987b683f-e05e-780a-40cf-68b6b8d0641f@redhat.com> (raw)
In-Reply-To: <70cb944b-269c-2969-4727-b889298ece30@linaro.org>

On 04/29/18 16:56, Richard Henderson wrote:
> On 04/27/2018 02:01 PM, Michal Suchánek wrote:
>> Is there any reason why the 64bit emulator would not run on 32bit
>> system? The emulated 64bit system is .. emulated after all.
> 
> It does run, but it requires that the 32-bit host perform double-word
> arithmetic to emulate the 64-bit guest.  When the 64-bit guest is really a
> 32-bit guest in disguise, this carries a performance penalty.
> 
> If we ever stop caring about 32-bit hosts, the question becomes moot.

What about guest RAM size (more precisely, guest-phys address space)?

The x86_64 target might want to use tens of GBs of guest-phys address
space, e.g. for cold-plugged RAM, for DIMM hotplug, for 64-bit PCI MMIO
aperture. To my understanding, all of those have to be expressed with
host virtual addresses in the QEMU process (regardless of TCG vs. KVM).
But on a 32-bit host, the QEMU process only has 4GB HVA space.

Thanks
Laszlo