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



On 16.12.21 03:10, Stefano Stabellini wrote:
On Wed, 15 Dec 2021, Juergen Gross wrote:
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?


I can see where Julien is coming from: we have been trying to keep the
arm32 and arm64 ABIs identical since the beginning of the project. So,
like Julien, my preference would be to always return 32-bit on ARM, both
aarch32 and aarch64. It would make things simple.

The case of XENMEM_maximum_ram_page is interesting but it is not a
problem in reality because the max physical address size is only 40-bit
for aarch32 guests, so 32-bit are always enough to return the highest
page in memory for 32-bit guests.

You are aware that this isn't the guest's max page, but the host's?

And all of this is about a 32-bit guest on a 64-bit host.


Juergen

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