On 6/3/23 15:06, Daniel P. Berrangé wrote:
On Mon, Mar 06, 2023 at 02:48:16PM +0100, Thomas Huth wrote:
On 06/03/2023 10.27, Daniel P. Berrangé wrote:
On Mon, Mar 06, 2023 at 09:46:55AM +0100, Thomas Huth wrote:
[...] If a 32-bit CPU guest +environment should be enforced, you can switch off the "long mode" CPU +flag, e.g. with ``-cpu max,lm=off``.
I had the idea to check this today and this is not quite sufficient, [...] A further difference is that qemy-system-i686 does not appear to enable the 'syscall' flag, but I've not figured out where that difference is coming from in the code.
I think I just spotted this by accident in target/i386/cpu.c around line 637:
#ifdef TARGET_X86_64 #define TCG_EXT2_X86_64_FEATURES (CPUID_EXT2_SYSCALL | CPUID_EXT2_LM) #else #define TCG_EXT2_X86_64_FEATURES 0 #endif
Hmm, so right now the difference between qemu-system-i386 and qemu-system-x86_64 is based on compile time conditionals. So we have the burden of building everything twice and also a burden of testing everything twice.
If we eliminate qemu-system-i386 we get rid of our own burden, but users/mgmt apps need to adapt to force qemu-system-x86_64 to present a 32-bit system.
What about if we had qemu-system-i386 be a hardlink to qemu-system-x86_64, and then changed behaviour based off the executed binary name ?
ie if running qemu-system-i386, we could present a 32-bit CPU by default. We eliminate all of our double compilation burden still. We still have extra testing burden, but it is in a fairly narrow area, so does not imply x2 the testing burden just $small-percentage extra testing ? That would means apps/users would not need to change at all, but we still get most of the win we're after on the QEMU side
Essentially #ifdef TARGET_X86_64 would be change 'if (is_64bit) {...}' in a handful of places, with 'bool is_64bit' initialized in main() from argv[0] ?
That is what Alex suggested me to do with ARM binaries as a prototype of unifying 32/64-bit binaries, avoiding to break users scripts.