Re: Capabilities, Anybody?

Robert Finch wrote:

On 2024-03-13 11:53 a.m., MitchAlsup1 wrote:

Robert Finch wrote:
 

Can capabilities be applied to address ranges?

 That is a major thing that they provide.
 

Segmentation similar to the PowerPC 32-bit segmentation is being used in the current project. Where the upper address bits select a segment register which provides more address bits. I would like to use the same descriptors for capabilities and the address range segmentation.

 How would you handle 2 billion Capabilities in a single application ??
 Each of which have a range of 2 GB each ??? and each containing at
least 1 M Capabilities ????

I should have been a bit more clear maybe, it has taken time to gel in my head.

PowerPC-32 has only 16 segment registers. I think these could be extended to capabilities registers in the same manner as proposed for the FS, GS registers in x64. I wonder if there is any value in doing so though, since the address is a constant. I think it should already be known if it would exceed a bounds. The segment registers simply tack on 24-bits to the left side of the remaining address bits to generate a 52-bit virtual address. I think all a capability would do is provide a slightly different means to calculate the address.

In the past, capability machines wanted to use capabilities for all relocation and all protection. As long as this is the case, an applica-
tion has an unbounded need for capabilities.
You can grant this with limited capabilities (top 4-odd bits) only when
you have a means to load a new capability into a known <capability> base
register[i]. Since this is privileged data, either the specified function-
ality of this instruction is precisely specified and operates with access to GuestOS address space.....it is difficult to imagine how to add Hyper-
Vision on top of GuestOS supervision. {{Or do you intend to void Hypervisors?}}

I have a couple of cores I can experiment with adding capabilities. For my current project there are 32 segment registers.

******

I am wondering if the ‘R’ term in the CHERI concentrate expansion calc can be less than zero or if it is a modulo value. It is shown as B[13:11] – 1. I am assuming it can go negative and is not modulo.

How “open” is CHERI ? Can CHERI based code be posted?