Re: [Qemu-devel] [PATCH 0/2] make guest-base support mandatory for TCG backends

[Laurent Desnogues]

On Tue, Oct 9, 2012 at 3:28 PM, Richard Henderson <address@hidden> wrote:
> On 10/09/2012 06:16 AM, Peter Maydell wrote:
>> That is, we could drop CONFIG_USER_GUEST_BASE. Does anybody have
>> a practical use case for the --disable-guest-base configuration?
>
> Nope, because the backends are good at eliminating the overhead
> during tcg_target_qemu_prologue if guest_base turns out to be zero.

I had some difficulties getting the same code generated with
--enable-guest-base as the one generated with --disable-guest-base,
for ARM on x86_64.  One has to play with -B 0 and -R, and that was
not obvious, while --disable-guest-base was obvious for me :-).

Guest ARM, host x86_64.

$ ./arm-linux-user/qemu-arm -singlestep -d in_asm,out_asm
/work/qemu/Tests/arm/gcc-a9-O3 x

IN:
0x00008138:  e59fc010      ldr  ip, [pc, #16]   ; 0x8150

OUT: [size=53]
0x602296c0:  mov    $0x8150,%ebp
0x602296c5:  mov    $0x2ac1f9b61000,%rdi
0x602296cf:  add    %rbp,%rdi
0x602296d2:  mov    (%rdi),%ebp
0x602296d4:  mov    %ebp,0x30(%r14)
0x602296d8:  jmpq   0x602296dd
0x602296dd:  mov    $0x813c,%ebp
0x602296e2:  mov    %ebp,0x3c(%r14)
0x602296e6:  mov    $0x2ac1f6760281,%rax
0x602296f0:  jmpq   0x6226d6b6

Running with -B 0 and playing with -R values until the program
succeeds in loading finally produces the same code as when
compiling QEMU with --disable-guest-base

OUT: [size=41]
0x602281a0:  mov    $0x8150,%ebp
0x602281a5:  mov    0x0(%rbp),%ebp
0x602281a8:  mov    %ebp,0x30(%r14)
0x602281ac:  jmpq   0x602281b1
0x602281b1:  mov    $0x813c,%ebp
0x602281b6:  mov    %ebp,0x3c(%r14)
0x602281ba:  mov    $0x2b9b26096281,%rax
0x602281c4:  jmpq   0x6226c1b6

I know the reason why reserved_va was set to the current value
(cf 288e65b9eea0c9b3cbe21be46f3e24e4e8b2a090), but I
thought it was interesting to point out the above.


Laurent