Re: [Xen-devel] [PATCH] RFC x86/hvm: Don't truncate the hvm hypercall index before range checking it

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 14.10.16 at 17:51, <andrew.cooper3@xxxxxxxxxx> wrote:
> When the compat hypercall ABI was added for HVM guests (i.e. supporting 32bit
> operating systems making hypercalls against a 64bit Xen), an ABI breakage was
> introduced for non-compat guests, as the 64bit hypercall index became
> truncated to 32 bits.
> 
> This has been the case for a very long time, but is not very obvious from the
> code, and definitely counterintuitive, seeing as all other 64bit parameters
> are passed without truncation.
> 
> However, the only supported method of making hypercalls is to call into the
> hypercall page, which in practice means that only hypercall index up to 63 are
> supported.
> 
> Therefore, take the opportunity to fix the ABI before it becomes impossible to
> fix.

Considering

    if ( (eax & 0x80000000) && is_viridian_domain(currd) )
        return viridian_hypercall(regs);

I'm not convinced we should change current behavior, the more that
the change has at least theoretical potential of breaking existing guests.

> @@ -4283,6 +4283,8 @@ int hvm_do_hypercall(struct cpu_user_regs *regs)
>          break;
>      }
>  
> +    eax = (mode == 8) ? regs->eax : regs->_eax;

But if we indeed want to make the adjustment, please use regs->rax
here (slightly helping the register field renaming series I have pending
for 4.9).

Jan

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