Re: memaccess: skipping mem_access_send_req

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 10/04/15 16:04, Gareth Stockwell wrote:
>
> My understanding of memaccess is that it provides the following
> functionality:
>
>  
>
> 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.
>
>  
>
> 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:
>
>  
>
> 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.
>
>  
>
> (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.
>
>  
>
> Does this sound reasonable?  Or is there a better way of modifying
> access permissions for a specific pfn range of a target domain?
>

Are you perhaps looking for something similar to Intel #VE support? 
(although I guess you are looking at ARM rather than x86)

It is not safe to always re-inject back into the vcpu, especially if the
permission changes were around the guest kernel stack, but a restricted
subset ought to be safe to bounce straight in.

~Andrew

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