[Xen-devel] Re: The memory type of non-RAM address is WB by default?

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

The issue with setting the WB/UC boundary at top of RAM rather than at 3.75G
is that this boundary is general no more than page-aligned. Hence you'd need
tens of variable MTRR ranges to express it; considerably more than the eight
which we supply. This is because of the requirement for each such range to
be power-of-two sized and aligned.

 -- Keir

On 23/10/08 10:49, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

> Why take any notice of guest MTRR/PAT in cases where the host knows better?
> 
>  -- Keir
> 
> On 23/10/08 10:36, "Cui, Dexuan" <dexuan.cui@xxxxxxxxx> wrote:
> 
>> In changeset 17471:cd5dc735bdf3(x86, hvm: Lots of MTRR/PAT emulation
>> cleanup), 
>> the default memory type is set to WB and the type of each special memory
>> range 
>> is set properly. Specially, [3.75G, 4G) is assumed as PCI space and marked as
>> UC.
>> 
>> I meet with an issue:
>> When I "xm pci-attach" a NIC into RHEL5.1 Linux guest(memory=256M), the
>> virtual MMIO BAR assigned by guest starts from 0x20000000 and I don't find
>> guest kernel tries to set the MTRR registers at all; as a result, the type of
>> the pfn is WB, but in hypervisor, the host mfn is UC, so when guest accesses
>> the vBAR, there are many lines of such warnings printed:
>> 
>> (XEN) mtrr.c:400:d10 Conflict occurs for a given guest l1e flags:163 at
>> 20000000 (the effective mm type:6), because the host mtrr type is:0
>> 
>> Here I think the device should work normally since the actual host mfn  is
>> UC, 
>> but we might as well fix such inconsistency.
>> Can't we set the default type to UC?
>> 
>> BTW: when I statically assign a NIC to guest via config file, since the vBAR
>> assigned by hvmloader starts from 0xF0000000, I don't see the warnings.
>> 
>> Thanks,
>> -- Dexuan
>> 
>> 
>> 



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