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Re: [Qemu-arm] [PATCH v2 0/4] hw/arm/boot: handle large Images more grac

From: Mark Rutland
Subject: Re: [Qemu-arm] [PATCH v2 0/4] hw/arm/boot: handle large Images more gracefully
Date: Fri, 7 Jun 2019 15:07:21 +0100
User-agent: Mutt/1.11.1+11 (2f07cb52) (2018-12-01)

Hi Peter,

Sorry for the delay in replying to this...

On Thu, May 16, 2019 at 03:47:29PM +0100, Peter Maydell wrote:
> This patchset attempts to fix https://bugs.launchpad.net/qemu/+bug/1823998
> which reports that we don't handle kernels larger than 128MB
> correctly, because we allow the initrd to be placed over the
> tail end of the kernel. AArch64 kernel Image files (since v3.17)
> report the total size they require (including any BSS area that
> isn't in the Image itself), so we can use that to be sure we
> place the initrd sufficiently far into the RAM.
> Patches 1 and 2 are new since v1; patches 3 and 4 are the old
> patches 1 and 2 (and are basically unchanged since v1).
> Patches 1 and 2 in this series are new. Patch 1 fixes bugs
> in the existing code where we were assuming that we could
> treat info->ram_size as the address of the end of RAM, which
> isn't true if the RAM doesn't start at address 0. (This
> generally went unnoticed thanks to the magic of unsigned integer
> underflow turning end-start calculations into very large max_size
> values for load_ramdisk_as() and friends.)
> Patch 2 adds some explicit checks that we don't try to put things
> entirely off the end of RAM (which avoids those accidental
> underflows).
> Patch 3 in this series adjusts our "where do we put the initrd"
> heuristic so that it always places it at least after whatever
> our best guess at the kernel size is. (This might still not
> be right for images like self-decompressing 32-bit kernels, where
> there's no way to know how big the kernel will be after
> decompression.)
> Patch 4 makes load_aarch64_image() return the
> kernel size as indicated in the Image file header, so that for
> the specific case of AArch64 Image files we will definitely not
> put the initrd on top of them.

With all 4 patches applied, I'm able to boot kernels with large BSS
segments (~128M, ~512M, and ~1G), and I get sensible warnings when they
are impossible to boot, e.g.

# 124M of RAM
[mark@gravadlaks:~/repro]% ./vmboot.sh ~/Image.test-128M
qemu-system-aarch64: kernel '/home/mark/Image.test-128M' is too large to fit in 
RAM (kernel size 155500544, RAM size 130023424)

# 150M of RAM
[mark@gravadlaks:~/repro]% ./vmboot.sh ~/Image.test-128M
qemu-system-aarch64: Not enough space for DTB after kernel/initrd

So feel free to add:

Tested-by: Mark Rutland <address@hidden>

Thanks for putting this together; this is _really_ useful for my testing
setup, and the warnings above are likely to save people a lot of
head-scratching in future. It would be great to see this merged. :)


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