Re: [BACKPORT 3.14.y] MIPS: KVM: Fix modular KVM under QEMU

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On Wed, 2016-07-13 at 18:14 +0100, James Hogan wrote:
> commit 797179bc4fe06c89e47a9f36f886f68640b423f8 upstream.
> 
> Copy __kvm_mips_vcpu_run() into unmapped memory, so that we can never
> get a TLB refill exception in it when KVM is built as a module.
> 
> This was observed to happen with the host MIPS kernel running under
> QEMU, due to a not entirely transparent optimisation in the QEMU TLB
> handling where TLB entries replaced with TLBWR are copied to a separate
> part of the TLB array. Code in those pages continue to be executable,
> but those mappings persist only until the next ASID switch, even if they
> are marked global.
> 
> An ASID switch happens in __kvm_mips_vcpu_run() at exception level after
> switching to the guest exception base. Subsequent TLB mapped kernel
> instructions just prior to switching to the guest trigger a TLB refill
> exception, which enters the guest exception handlers without updating
> EPC. This appears as a guest triggered TLB refill on a host kernel
> mapped (host KSeg2) address, which is not handled correctly as user
> (guest) mode accesses to kernel (host) segments always generate address
> error exceptions.
> 
> Signed-off-by: James Hogan <james.hogan@xxxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx>
> Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: linux-mips@xxxxxxxxxxxxxx
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> [james.hogan@xxxxxxxxxx: backported for stable 3.14]
> Signed-off-by: James Hogan <james.hogan@xxxxxxxxxx>
[...]

Belatedly queued this up for 3.16.

Ben.

-- 
Ben Hutchings
compatible: Gracefully accepts erroneous data from any source

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