Re: [PATCH v3 02/13] xen: harmonize return types of hypercall handlers

[Juergen Gross <jgross@xxxxxxxx>]

On 14.12.21 18:36, Julien Grall wrote:
> Hi,
>
> On 08/12/2021 15:55, Juergen Gross wrote:
> > Today most hypercall handlers have a return type of long, while the
> > compat ones return an int. There are a few exceptions from that rule,
> > however.
> So on Arm64, I don't think you can make use of the full 64-bit because a 
> 32-bit domain would not be able to see the top 32-bit.
> In fact, this could potentially cause us some trouble (see [1]) in Xen.
> So it feels like the hypercalls should always return a 32-bit signed 
> value on Arm.
This would break hypercalls like XENMEM_maximum_ram_page which are able
to return larger values, right?

> The other advantage is it would be clear that the top 32-bit are not 
> usuable. Stefano, what do you think?
Wouldn't it make more sense to check the return value to be a sign
extended 32-bit value for 32-bit guests in do_trap_hypercall() instead?

The question is what to return if this is not the case. -EDOM?

> > Get rid of the exceptions by letting compat handlers always return int
> > and others always return long.
> >
> > For the compat hvm case use eax instead of rax for the stored result as
> > it should have been from the beginning.
> >
> > Additionally move some prototypes to include/asm-x86/hypercall.h
> > as they are x86 specific. Move the do_physdev_op() prototype from both
> > architecture dependant headers to the common one. Move the
> > compat_platform_op() prototype to the common header.
> >
> > Switch some non style compliant types (u32, s32, s64) to style compliant
> > ones.
> TBH, I think this should have been split because the modifications are 
> done on lines that are untouched.
> The extra patch would have made the review easier (even if this patch is 
> still quite small).
I can split the patch if you want.


Juergen

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