Re: [Xen-devel] [PATCH 0/2 for-4.12] Introduce runstate area registration with phys address

[Julien Grall <julien.grall@xxxxxxx>]

Hi Roger,

Thank you for the answer.

On 07/03/2019 16:16, Roger Pau Monné wrote:
> On Thu, Mar 07, 2019 at 03:17:54PM +0000, Julien Grall wrote:
> > Hi Andrii,
> >
> > On 07/03/2019 14:34, Andrii Anisov wrote:
> > > On 07.03.19 16:02, Julien Grall wrote:
> > > > >    - IMHO, this implementation is simpler and cleaner than what I
> > > > > have for runstate mapping on access.
> > > > Did you implement it using access_guest_memory_by_ipa?
> > > Not exactly, access_guest_memory_by_ipa() has no implementation for x86.
> > > But it is made around that code.
> > For the HVM, the equivalent function is hvm_copy_to_guest_phys. I don't know
> > what would be the interface for PV. Roger, any idea?
> For PV I think you will have to use get_page_from_gfn, check the
> permissions, map it, write and unmap it. The same flow would also work
> for HVM, so I'm not sure if there's much point in using
> hvm_copy_to_guest_phys. Or you can implement a generic
> copy_to_guest_phys helper that works for both PV and HVM.
>
> Note that for translated guests you will have to walk the HAP page
> tables for each vCPU for each context switch, which I think will be
> expensive in terms of performance (I might be wrong however, since I
> have no proof of this).
AFAICT, we already walk the page-table with the current implementation. So this 
should be no different here, except we will not need to walk the guest-PT here. No?
Cheers,

--
Julien Grall

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