Messages in this thread | | | Subject | Re: [PATCH v12 11/20] KVM: xen: allow shared_info to be mapped by fixed HVA | From | David Woodhouse <> | Date | Thu, 08 Feb 2024 09:26:25 -0800 |
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On Thu, 2024-02-08 at 16:51 +0000, Paul Durrant wrote: > On 08/02/2024 16:48, Sean Christopherson wrote: > > On Thu, Feb 08, 2024, Paul Durrant wrote: > > > With the benefit of some sleep, I'm wondering why 0 is a 'ridiculous' > > > invalid value for a *virtual* address? Surely it's essentially a numerical > > > cast of the canonically invalid NULL pointer? > > > > It's legal to mmap() virtual address '0', albeit not by default:
Well yes, to make dosemu work. But if you attempt to actually *do* that in C code, the compiler itself doesn't cope...
$ cat foo.c int foo(int *bar) { if (bar) return 0; return *bar; } $ gcc -O2 -S -o- foo.c .. foo: LFB0: .cfi_startproc endbr64 testq %rdi, %rdi je .L4 xorl %eax, %eax ret .p2align 4,,10 .p2align 3 L4: movl 0, %eax ud2 .cfi_endproc LFE0: .size foo, .-foo
Note the ud2 instead of actually trying to dereference it.
Using anything except NULL as the "no value" value doesn't make sense to me. It violates the principle of least surprise and would be a really bad API. [unhandled content-type:application/pkcs7-signature] | |