Re: kexec and xen/arch/x86/boot/head.S trampoline

[Trammell Hudson <hudson@trmm.net>]

On Wed, Feb 28, 2018 at 03:07:41PM +0000, Andrew Cooper wrote:
> On 28/02/18 14:08, Trammell Hudson wrote:
> > kexec allocates a 1 page segment at 0x0 and memsets most of it to zero,
> > wiping out coreboot's EBDA structure, which xen's head.S consulted to
> > allocate the trampoline.  Xen's code looks like this:
> 
> This sounds like a bug as and of itself.  I presume this is to do with
> IVT handling?

Maybe?  That's the right address and it only populates 0x78 bytes
(in qemu) with contents, which would be about right for the interrupt
table.  I haven't tracked down where that data comes from.

> [...]
> > Are there reasons to prefer EBDA over mbi->mem_lower?
> 
> I think the expectation was that the EBDA would be more reliable than
> mbi->mem_lower.  However, we recently hit a similar bug with PVH
> handling (c/s a232346b1fe), and these days, the EBDA is quite likely not
> to be present.
> 
> I think we probably can use mbi->mem_lower (if available and sane) by
> default.  If the bootloader has messed that up, all bets are off
> anyway.  Irrespective, we should fix the EBDA lower sanity check.

What do you think of something like this instead?

        /*
         * If the multiboot structure memory limits are available,
         * use it for the trampoline.
         */
        testb   $MBI_MEMLIMITS,(%ebx)
        jz      1f                  /* not available? BDA value will be fine */
        mov     MB_mem_lower(%ebx),%ecx
        cmp     $0x100,%ecx         /* is the multiboot value too small? */
        jae     2f

1:
        /* MBI not available, set up trampoline segment 64k below EBDA */
        movzwl  0x40e,%ecx          /* EBDA segment */
        cmp     $0xa000,%ecx        /* sanity check (high) */
        jae     0f
        cmp     $0x4000,%ecx        /* sanity check (low) */
        jae     2f
0:
        movzwl  0x413,%ecx          /* use base memory size on failure */
        cmp     $0xa000,%ecx        /* sanity check (high) */
        jae     bad_ebda
        cmp     $0x4000,%ecx        /* sanity check (low) */
        jb      bad_ebda

2:      /* Reserve 64kb for the trampoline */
        shl     $10-4,%ecx          /* convert to bytes */
        sub     $0x1000,%ecx

-- 
Trammell

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