Re: Physical memory read: word crosses page boundary + host kernel oops

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Kiselev, Sergey wrote:
> Hi,
>  
> 1. When booting old Linux (RH7.1 based, 2.4.9, 32bit) guest on kvm-18, 
> kvm userspace process crashes with 'Bus error' (last output on guest's 
> screen is "Uncompressing Linux...").
> I did some debugging and found that kvm_readl() function calls 
> ldl_phys() with address 0x9FFFD, so resulting double-word read crosses 
> page boundary.
> After looking at qemu/exec.c it seems that ld*_phys and st*_phys 
> functions not really care about crossing page boundary (even there is 
> a comment sayng "warning: addr must be aligned"). So either 
> qemu/exec.c should be updated to check such condition or (more logical 
> place) qemu/qemu-kvm.c should take care of it.
>  
> gdb backtrace:
> (gdb) bt
> #0  ldl_phys (addr=4093) at ../cpu-all.h:322
> #1  0x000000000047e08d in kvm_readl (opaque=0x9f, addr=159,
>     data=0x2b63605a5030) at /srv/src/kvm-18/qemu/qemu-kvm.c:543

This is quite surprising.  I agree that hacking kvm_readl() is the best fix.


> 2. After working-around the first issue, I have following problem: at 
> some point of guest's Linux boot sequence (after running 
> microcode_ctl, before running kudzu) following oops happens:
>
>
> Mar 27 12:10:39 itstl140 kernel: Code: 4c 8b 08 41 0f 18 09 48 8d 70 
> d8 31 c0 e9 39 ff ff ff 48 63
> Mar 27 12:10:39 itstl140 kernel: RIP 
> <ffffffff88366aa6>{:kvm:mmu_page_remove_parent_pte+225} RSP 
> <ffff81014e34f938>
> Mar 27 12:10:39 itstl140 kernel: CR2: 000000030593a563
> I tried to disable both microcode_ctl and kudzu, in this case oops h
Strangely, I've seen this exact oops somewhere booting Windows XP in 
safe mode.  I haven't been able to reproduce it, though.

If this is reproducible, it may be debugged by turning on audit 
(s/#undef AUDIT/#define AUDIT/ in mmu.c).  Audit slows the guest down, 
but is a little faster if you reduce the amount of guest memory.  If 
this is reproducible using a publicly available image, I may have a go 
at it too.


-- 
error compiling committee.c: too many arguments to function


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