On 09/18/2018 08:44 PM, Peter Maydell wrote: > On 31 August 2018 at 11:38, Cédric Le Goater <c...@kaod.org> 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?
It is true that the user does not care about internals. "execute-from-flash" as you proposed below ? >> 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 ? That is what it seems but I haven't dug further the problem. The mainline U-Boot also has a rewritten DRAM calibration. Using the logs is too painful so I would need to add some statistics on what is being done. May be number of read/write ops on the flash model that we could dump from the monitor, or some more low-level figures of QEMU internals. >> 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. Yes that is a good a point. When performance is fixed, we should remove the ROM region and the machine option. What I am proposing is a more of a work around to analyze a problem than something we would keep in the future. Thanks, C.
