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Re: [PATCH 00/18] KVM/MIPS32: Support for the new Virtualization ASE (VZ

To: Sanjay Lal <>
Subject: Re: [PATCH 00/18] KVM/MIPS32: Support for the new Virtualization ASE (VZ-ASE)
From: David Daney <>
Date: Mon, 20 May 2013 10:29:13 -0700
Cc:,, Ralf Baechle <>, Gleb Natapov <>, Marcelo Tosatti <>
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On 05/20/2013 09:58 AM, Sanjay Lal wrote:

On May 20, 2013, at 8:50 AM, David Daney wrote:

On 05/18/2013 10:47 PM, Sanjay Lal wrote:
The following patch set adds support for the recently announced virtualization
extensions for the MIPS32 architecture and allows running unmodified kernels in
Guest Mode.

For more info please refer to :
        MIPS Document #: MD00846
        Volume IV-i: Virtualization Module of the MIPS32 Architecture

which can be accessed @:

The patch is agains Linux-3.10-rc1.

KVM/MIPS now supports 2 modes of operation:

(1) VZ mode: Unmodified kernels running in Guest Mode.  The processor now 
     an almost complete COP0 context in Guest mode. This greatly reduces VM 

Two questions:

1) How are you handling not clobbering the Guest K0/K1 registers when a Root 
exception occurs?  It is not obvious to me from inspecting the code.

2) What environment are you using to test this stuff?

David Daney

(1) Newer versions of the MIPS architecture define scratch registers for just 
this purpose, but since we have to support standard MIPS32R2 processors, we use 
the DDataLo Register (CP0 Register 28, Select 3) as a scratch register to save 
k0 and save k1 @ a known offset from EBASE.

Right, I understand that. But I am looking at arch/mips/mm/tlbex.c, and I don't see the code that does that for TLBRefill exceptions.

Where is it done for interrupts? I would expect code in arch/mips/kernel/genex.S and/or stackframe.h would handle this. But I don't see where it is.

Am I missing something?

David Daney

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