Hello Roger, Sorry for a delayed response.
On 07.02.19 12:35, Roger Pau Monné wrote:
I've been thinking about this with other Citrix folks, and I'm not sure the proposed patch is a good solution. It's not possible for us to know whether there's a kernel somewhere relying on changing the virtual address of the runtime state area without issuing a new hypercall.
Do you mean allocating another buffer by VM and mapping it to a virtual address known to XEN? Or remapping existing buffer to a different virtual address?
If such kernel existed by making this change we would introduce random memory corruption to that kernel, which would be very hard to track and considered a regression.
I guess you actually mean that VM is trying to map another physical buffer to a vaddr known to XEN. As I said here [1], even current implementation looks problematic, because VM's changes in PT are not atomic from the hypervisor point of view. I stated that for ARM, but x86 does not seem to differ here. Actually VM trying to make changes behind a hypervisor's back is a really bad idea. Because the hypervisor *is* always behind the VM's back.
I think the best way to move forward is to pick my patch and introduce a new hypercall that instead of a virtual address takes a guest physical address. Will you be OK with this Andrii?
It might work better for this. And introducing a new interface is a chance to get rid of a mixed legacy.
Note that the Linux kernel would also need to be modified to make use of this new hypercall, but that's likely close to a 1 line change.I would not say it is a 1 line change.
I think about a page alignment for the runstate area, I'd like to have it directly accessed from XEN. I do not like `update_runstate_area()` with its all kind of copy_to_guest, done twice a context switch. [1] https://lists.xenproject.org/archives/html/xen-devel/2019-02/msg00164.html -- Sincerely, Andrii Anisov. _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
