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Re: Proposal for physical address based hypercalls
- To: Juergen Gross <jgross@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 29 Sep 2022 14:58:45 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Delivery-date: Thu, 29 Sep 2022 12:58:54 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 29.09.2022 14:26, Juergen Gross wrote:
> On 29.09.22 13:32, Jan Beulich wrote:
>> Finally - in how far are we concerned of PV guests using linear
>> addresses for hypercall buffers? I ask because I don't think the model
>> lends itself to use also for the PV guest interfaces.
>
> Good question.
>
> As long as we support PV guests we can't drop support for linear addresses
> IMO. So the question is whether we are fine with PV guests not using the
> pre-registered buffers, or if we want to introduce an interface for PV
> guests using GFNs instead of MFNs.
GFN == MFN for PV, and using PFN space (being entirely controlled by the
guest) doesn't look attractive either. Plus any form of translation we'd
need to do for PV would involve getting and putting page references (for
writes also type references), along the lines of what is already
happening for HVM. Since "put" may involve freeing a page, which in turn
require locks to be taken, we'd need to carefully check that no such
translation can occur from an inappropriate call chain.
Jan
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