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From: | Anthony Liguori |
Subject: | Re: [Qemu-devel] [PATCH v2] arm: add device tree support |
Date: | Wed, 01 Feb 2012 07:52:24 -0600 |
User-agent: | Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.23) Gecko/20110922 Lightning/1.0b2 Thunderbird/3.1.15 |
On 02/01/2012 07:49 AM, Alexander Graf wrote:
On 01.02.2012, at 14:44, Anthony Liguori wrote:On 02/01/2012 07:32 AM, Alexander Graf wrote:On 01.02.2012, at 14:25, Anthony Liguori wrote:On 02/01/2012 07:10 AM, Peter Maydell wrote:On 1 February 2012 13:04, Anthony Liguori<address@hidden> wrote:How does it race? Devices normally never touch memory so a loader device will be the only thing mucking with memory.The obvious one is "loader reset function wants to set starting PC to entry point of kernel/etc" vs "CPU device reset wants to set starting PC to hardware-mandated reset vector". We have this at the moment, of course, and I think we implicitly rely on reset handlers being called in order of registration...I'm a bit confused, why can't the kernel loader be implemented in terms of a firmware blob? This is what we do for x86 and it solves this problem robustly. Isn't it just a matter of a few instructions to do a jmp to a known location?Only if you have non-semi-hosted modes. For e500 for example, we don't have a bios flash region mapped through mmio available. So we would have to write the "jump to kernel" code into ram. But where in RAM? Linux starts at address 0, so that one's taken.The processor has to have a defined sequence where IP is fixed to a specific value, no? How else would the real hardware bootstrap software?Real hardware boots u-boot which initializes lots of things and then goes into the actual booting of Linux. Today, we're doing semi-hosting though, without u-boot. We just directly boot into Linux.
Fine, but to boot u-boot, the real hardware must set IP to something that's most likely an offset into ROM flash.
Why can't we bootstrap semi-hosted mode by having a ROM somewhere that just redirects IP?
It doesn't have to be a full blown u-boot.
That's why I'm saying things don't work out all that simple with semi-hosted environments. Now you could argue that semi-hosting is a bad thing, but we'll always have to have it. On s390 for example, semi-hosting is how real hardware works. Or at least the parts that are visible to end users. Especially when you model PV machines, you'll have a hard time with fixed reset IPs too.
s390 is a special case because "real hardware" is not actually real hardware. It's a VM.
Regards, Anthony Liguori
However, couldn't we model some wiring that allows our dash-kernel-boot-device to override the reset vector on CPUs? Alex
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