Re: [Qemu-devel] [PATCH v2] PAM: make PAM emulation closer to documentation

[Paolo Bonzini <pbonzini@redhat.com>]

On 09/09/2015 14:03, Ефимов Василий wrote:
> We also can set up redirection at machine initialization, but using of
> listener makes sure redirection is actual at runtime. I do not know
> case in which regions at the PAM addresses are changed dynamically
> during guest work. But even during machine initialization the memory
> tree is changed multiple times. So, listener at least ensures the last
> version of tree is used for redirection choice.

Fair enough.

>> 2) For the "Make current PAM region an I/O region" case, you could add
>> an IOMMU region that to 0xc0000-0xfffff.  The listener would disable
>> pam->region[1] if address_space_translate returns an I/O region and
>> enable it if it returns RAM/ROM.
> As I see, IOMMU cannot be used for CPU access to memory because of
> assertion (!section->mr->iommu_ops) in
> exec.c: address_space_translate_for_iotlb
> IOMMU is used for redirection of device and debug access through
> address_space_translate. Please correct me if I miss something.

You're right.  We could remove the assertion and reuse subpage_ops for
IOMMUs, it would not allow running code but it would allow accesses.
But it's not necessary because this can never happen in practice.

> There is no known case in which an I/O region is at PAM addresses.
> But it is theoretically possible. Hence, I decide to implement it
> instead of insert an assertion.

Let's keep the code simple and assert.  If you put a BAR at
0xc0000-0xfffff, RAM wins, so this situation should never happen on x86
chipsets.

Paolo