On 1 April 2023 12:57:33 BST, Michael Tokarev <m...@tls.msk.ru> wrote:
>01.04.2023 14:45, Joao Martins пишет:
>> On 01/04/2023 09:40, Michael Tokarev wrote:
>>> After bringing in xen guest support, qemu fails to build on x32:
>>> Adding the folks who added the feature too
>>
>>> target/i386/kvm/xen-emu.c:876:5: note: in expansion of macro
>>> ‘qemu_build_assert’
>>> 876 | qemu_build_assert(sizeof(struct vcpu_info) == 64);
>>> | ^~~~~~~~~~~~~~~~~
>>>
>>> This one should be easy to fix, but I wonder if there are other issues
>>> with x32 exists..
>>>
>> Not sure.
>>
>> struct vcpu_info is supposed to be 64bytes on both 32-bit and 64-bit builds.
>>
>> If anything maybe arch_vcpu_info struct is different on 32-bit and 64-bit...
>> David, Paul, any ideas?
>
>Yes, it is arch_vcpu_info. I assumed it is a trivial thing, but let me explain
>if it's not the case.
>
>include/hw/xen/interface/arch-x86/xen.h :
>
>#if defined(__i386__)
>#include "xen-x86_32.h"
>#elif defined(__x86_64__)
>#include "xen-x86_64.h"
>#endif
>
>(I wonder if it ever possible to have none of the two defined).
>
>Now, xen-x86_32.h has:
>
>struct arch_vcpu_info {
> unsigned long cr2;
> unsigned long pad[5]; /* sizeof(vcpu_info_t) == 64 */
>};
>
>Assuming sizeof(long)==32bits. But actually it is 64bits on x32.
>
>While xen-x86_64.h has:
>
>struct arch_vcpu_info {
> unsigned long cr2;
> unsigned long pad; /* sizeof(vcpu_info_t) == 64 */
>};
>
>
>It looks like for x32, the test in arch-x86/xen.h should be:
>
>#if defined(__x86_64__)
>#include "xen-x86_64.h"
>#else
>#include "xen-x86_32.h"
>#endif
>
>since x32 is almost like x86_64. The only difference from x86_64
>is sizeof(pointer), which is 32bits.
Hm, doesn't x32 also align uint64_t to 64 bits (unlike i386 which only aligns
it to 32 bits)?
>Well. Maybe xen support should be disabled entirely on x32.
>Or maybe x32 should be declared as unsupported entirely.
>I dunno.
We rely heavily on the struct layouts being precisely the same, since these are
ABI for the Xen guests. Wherever there was some 32/64 compatibility issue — and
often where there wasn't — that's why I littered it with those build
assertions.
But while there are enough in there to sanity check the i386 vs. x86_64 ABI
differences, I wouldn't swear that I've put in enough checks for x32. So "it
builds without hitting an assertion" does not necessarily mean it'll be
*correct*. Let's disable it on x32.
