Re: [PATCH] powerpc/kvm: Fix magic page vs. 32-bit RTAS on ppc64

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2012-03-14 at 18:52 +1100, Benjamin Herrenschmidt wrote:
> When the kernel calls into RTAS, it switches to 32-bit mode. The
> magic page was is longer accessible in that case, causing the
> patched instructions in the RTAS call wrapper to crash.
> 
> This fixes it by making available a 32-bit mapping of the magic
> page in that case. This mapping is flushed whenever we switch
> the kernel back to 64-bit mode.

I forgot to give credit to Alex for the original patch, which I tweaked
a little bit (among others it was missing the bit in kvmppc_gfn_to_pfn)

Cheers,
Ben.

> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> ---
> 
> Avi, please consider merging ASAP as this is a fairly annoying
> bug and the fix is reasonably obvious.
> 
>  arch/powerpc/kvm/book3s.c    |    3 +++
>  arch/powerpc/kvm/book3s_pr.c |   17 +++++++++++++++++
>  2 files changed, 20 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
> index e41ac6f..34487d4 100644
> --- a/arch/powerpc/kvm/book3s.c
> +++ b/arch/powerpc/kvm/book3s.c
> @@ -289,6 +289,9 @@ pfn_t kvmppc_gfn_to_pfn(struct kvm_vcpu *vcpu, gfn_t gfn)
>  {
>  	ulong mp_pa = vcpu->arch.magic_page_pa;
>  
> +	if (!(vcpu->arch.shared->msr & MSR_SF))
> +		mp_pa = (uint32_t)mp_pa;
> +
>  	/* Magic page override */
>  	if (unlikely(mp_pa) &&
>  	    unlikely(((gfn << PAGE_SHIFT) & KVM_PAM) ==
> diff --git a/arch/powerpc/kvm/book3s_pr.c b/arch/powerpc/kvm/book3s_pr.c
> index d6851a1..23919d4 100644
> --- a/arch/powerpc/kvm/book3s_pr.c
> +++ b/arch/powerpc/kvm/book3s_pr.c
> @@ -137,6 +137,20 @@ void kvmppc_set_msr(struct kvm_vcpu *vcpu, u64 msr)
>  		}
>  	}
>  
> +	/*
> +	 * When switching from 32 to 64-bit, we may have a stale 32-bit
> +	 * magic page around, we need to flush it. Typically 32-bit magic
> +	 * page will be instanciated when calling into RTAS. Note: We
> +	 * assume that such transition only happens while in kernel mode,
> +	 * ie, we never transition from user 32-bit to kernel 64-bit with
> +	 * a 32-bit magic page around.
> +	 */
> +	if (!(old_msr & MSR_PR) && !(old_msr & MSR_SF) && (msr & MSR_SF)) {
> +		/* going from RTAS to normal kernel code */
> +		kvmppc_mmu_pte_flush(vcpu, (uint32_t)vcpu->arch.magic_page_pa,
> +				     ~0xFFFUL);
> +	}
> +
>  	/* Preload FPU if it's enabled */
>  	if (vcpu->arch.shared->msr & MSR_FP)
>  		kvmppc_handle_ext(vcpu, BOOK3S_INTERRUPT_FP_UNAVAIL, MSR_FP);
> @@ -242,6 +256,9 @@ static int kvmppc_visible_gfn(struct kvm_vcpu *vcpu, gfn_t gfn)
>  {
>  	ulong mp_pa = vcpu->arch.magic_page_pa;
>  
> +	if (!(vcpu->arch.shared->msr & MSR_SF))
> +		mp_pa = (uint32_t)mp_pa;
> +
>  	if (unlikely(mp_pa) &&
>  	    unlikely((mp_pa & KVM_PAM) >> PAGE_SHIFT == gfn)) {
>  		return 1;