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Re: [Qemu-arm] [PATCH 04/11] hw/arm/aspeed: add a 'mmio-exec' property t

From: Peter Maydell
Subject: Re: [Qemu-arm] [PATCH 04/11] hw/arm/aspeed: add a 'mmio-exec' property to boot from the FMC flash module
Date: Tue, 18 Sep 2018 19:44:22 +0100

On 31 August 2018 at 11:38, Cédric Le Goater <address@hidden> wrote:
> Now that MMIO execution is supported, introduce a 'mmio-exec' property
> to boot directly from CE0 of the FMC controller using a memory region
> alias.

The name of this property seems to be a reference to QEMU's
internals: is there some other name that would make more sense
to a user who cares more about the guest and the emulated hardware?

> The overhead for the current firmware images using a custom U-Boot is
> around 2 seconds, which is fine, but with a U-Boot from mainline, it
> takes an extra 50 seconds or so to reach Linux. This might be related
> to the fact that a device tree is used.

Is this overhead just because we do a lot of execution from the
mmio region and that's slower than execution from RAM ?

> MMIO execution is not activated by default because until boot time is
> improved.

Can the guest tell the difference? I'm wondering how much we
might need to care about backward compatibility if in future
we fix the performance problem, or if we can just switch the
default to execute-from-flash without breaking guests.

-- PMM

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