Re: [Xen-devel] RMRR Fix Design for Xen

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Fri, Dec 19, 2014 at 1:21 AM, Tiejun Chen <tiejun.chen@xxxxxxxxx> wrote:
>                     RMRR Fix Design for Xen
>
> This design is a goal to fix RMRR for Xen. It includes four sectors as
> follows:
>
>     * Background
>     * What is RMRR
>     * Current RMRR Issues
>     * Design Overview
>
> We hope this can help us to understand current problem then figure out a
> clean and better solution everyone can agree now to go forward.
>
> Background
> ==========
>
> We first identified this RMRR defect when trying to pass-through IGD device,
> which can be simply fixed by adding an identity mapping in case of shared
> EPT table. However along with the community discussion, it boiled down to
> a more general RMRR problem, i.e. the identity mapping is brute-added
> in hypervisor, w/o considering whether conflicting with an existing guest
> PFN ranges. As a general solution we need invent a new mechanism so
> reserved ranges allocated by hypervisor can be exported to the user space
> toolstack and hvmloader, so conflict can be detected when constructing
> guest PFN layout, with best-effort avoidance policies to further help.
>
> What is RMRR
> ============
>
> RMRR is a acronym for Reserved Memory Region Reporting.
>
> BIOS may report each such reserved memory region through the RMRR structures,
> along with the devices that requires access to the specified reserved memory
> region. Reserved memory ranges that are either not DMA targets, or memory
> ranges that may be target of BIOS initiated DMA only during pre-boot phase
> (such as from a boot disk drive) must not be included in the reserved memory
> region reporting. The base address of each RMRR region must be 4KB aligned and
> the size must be an integer multiple of 4KB. BIOS must report the RMRR 
> reported
> memory addresses as reserved in the system memory map returned through methods
> suchas INT15, EFI GetMemoryMap etc. The reserved memory region reporting
> structures are optional. If there are no RMRR structures, the system software
> concludes that the platform does not have any reserved memory ranges that are
> DMA targets.
>
> The RMRR regions are expected to be used for legacy usages (such as USB, UMA
> Graphics, etc.) requiring reserved memory. Platform designers shouldavoid or
> limit use of reserved memory regions since these require system software to
> create holes in the DMA virtual address range available to system software and
> its drivers.
>
> The following is grabbed from my BDW:
>
> (XEN) [VT-D]dmar.c:834: found ACPI_DMAR_RMRR:
> (XEN) [VT-D]dmar.c:679:   RMRR region: base_addr ab80a000 end_address ab81dfff
> (XEN) [VT-D]dmar.c:834: found ACPI_DMAR_RMRR:
> (XEN) [VT-D]dmar.c:679:   RMRR region: base_addr ad000000 end_address af7fffff
>
> Here USB occupies 0xab80a000:0xab81dfff, IGD owns 0xad000000:0xaf7fffff.
>
> Note there are zero or more Reserved Memory Region Reporting (RMRR) in one 
> given
> platform. And multiple devices may share one RMRR range. Additionally RMRR can
> go anyplace.

Tiejun,

Thanks for this document -- such a document is really helpful in
figuring out the best way to architect the solution to a problem.

I hope you don't mind me asking a few additional questions here.
You've said that:
* RMRR is a range used by devices (typically legacy devices such as
USB, but apparently also newer devices like IGD)
* RMRR ranges are reported by BIOSes
* RMRR ranges should be avoided by the guest.

I'm still missing a few things, however.

* In the case of passing through a virtual device, how does the
"range" apply wrt gpfn space and mfn space?  I assume in example
above, the range [ab80a000,ab81dfff] corresponds to an mfn range.
When passing through this device to the guest, do pfns
[ab80a000,ab81dfff] need to be mapped to the same mfn range (i.e., 1-1
mapping), or can they be mapped from somewhere else in pfn space?

* You've described the range, but later on you talk about Xen
"creating" RMRR mappings.  What does this mean?  Are there registers
that need to be written?  Do the ept / IOMMU tables need some kind of
special flags?

Thanks,
 -George

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