Re: [Xen-devel] memaccess: skipping mem_access_send_req

[Tamas K Lengyel <tklengyel@xxxxxxxxxxxxx>]

On Fri, Apr 10, 2015 at 5:04 PM, Gareth Stockwell <Gareth.Stockwell@xxxxxxx> wrote:

My understanding of memaccess is that it provides the following functionality:

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1. Modify permission values in the page table of the target domain.

2. When the domain generates an illegal access, the exception handler delegates to memaccess, which pauses the VCPU and records the event in a ring buffer.

3. Some permission values are special in that they automatically mutate to a different value following the first access; others are persistently applied.

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We would like to use memaccess to perform (1) - but rather than pausing the VCPU in (2), instead simply directly inject the exception into the VCPU. I can see two ways of doing this:

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a) Implement an observer of the ring buffer, which triggers injection of the exception and unpausing of the VCPU.

b) Define new xenmem_access_t values which cause the exception handler to reinject rather than adding a message to the ring buffer.

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(a) seems cumbersome, and requires multiple context switches in order to handle the exception.

(b) therefore looks preferable, and I think should be fairly simple to implement on top of https://github.com/tklengyel/xen/tree/arm_memaccess15.

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Does this sound reasonable? Or is there a better way of modifying access permissions for a specific pfn range of a target domain?

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Gareth

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Hi Gareth,

interesting use-case. It should be possible and I agree that option b is your easiest route to go forward with. FYI my arm_memaccess15 branch is still being worked on so it's not yet safe to branch off of it. I'm planning on submitting it in the very near future (maybe later today) and hopefully it will be merged before the end of the month ;)

Cheers,

Tamas

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