Subject: Re: [PATCH v3 29/33] KVM: PPC: Book3S HV: Handle differing endianness for H_ENTER_NESTED

From: David Gibson <[email-masked]>
Date: Wed, 3 Oct 2018 16:13:36 +1000

On Tue, Oct 02, 2018 at 09:31:28PM +1000, Paul Mackerras wrote:
> From: Suraj Jitindar Singh <[email-masked]>
> 
> The hcall H_ENTER_NESTED takes as the two parameters the address in
> L1 guest memory of a hv_regs struct and a pt_regs struct which the
> L1 guest would like to use to run a L2 guest and in which are returned
> the exit state of the L2 guest.  For efficiency, these are in the
> endianness of the L1 guest, rather than being always big-endian as is
> usually the case for PAPR hypercalls.

Does that actually make a difference for efficiency?  I thought the
presence of the byte-reversing loads and stores meant byteswapping was
basically zero-cost on POWER.

> When reading/writing these structures, this patch handles the case
> where the endianness of the L1 guest differs from that of the L0
> hypervisor, by byteswapping the structures after reading and before
> writing them back.
> 
> Since all the fields of the pt_regs are of the same type, i.e.,
> unsigned long, we treat it as an array of unsigned longs.  The fields
> of struct hv_guest_state are not all the same, so its fields are
> byteswapped individually.
> 
> Signed-off-by: Suraj Jitindar Singh <[email-masked]>
> Signed-off-by: Paul Mackerras <[email-masked]>

Looks correct, though, so

Reviewed-by: David Gibson <[email-masked]>

> ---
>  arch/powerpc/kvm/book3s_hv_nested.c | 51 ++++++++++++++++++++++++++++++++++++-
>  1 file changed, 50 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv_nested.c b/arch/powerpc/kvm/book3s_hv_nested.c
> index 7b1088a..228dc11 100644
> --- a/arch/powerpc/kvm/book3s_hv_nested.c
> +++ b/arch/powerpc/kvm/book3s_hv_nested.c
> @@ -51,6 +51,48 @@ void kvmhv_save_hv_regs(struct kvm_vcpu *vcpu, struct hv_guest_state *hr)
>  	hr->ppr = vcpu->arch.ppr;
>  }
>  
> +static void byteswap_pt_regs(struct pt_regs *regs)
> +{
> +	unsigned long *addr = (unsigned long *) regs;
> +
> +	for (; addr < ((unsigned long *) (regs + 1)); addr++)
> +		*addr = swab64(*addr);
> +}
> +
> +static void byteswap_hv_regs(struct hv_guest_state *hr)
> +{
> +	hr->version = swab64(hr->version);
> +	hr->lpid = swab32(hr->lpid);
> +	hr->vcpu_token = swab32(hr->vcpu_token);
> +	hr->lpcr = swab64(hr->lpcr);
> +	hr->pcr = swab64(hr->pcr);
> +	hr->amor = swab64(hr->amor);
> +	hr->dpdes = swab64(hr->dpdes);
> +	hr->hfscr = swab64(hr->hfscr);
> +	hr->tb_offset = swab64(hr->tb_offset);
> +	hr->dawr0 = swab64(hr->dawr0);
> +	hr->dawrx0 = swab64(hr->dawrx0);
> +	hr->ciabr = swab64(hr->ciabr);
> +	hr->hdec_expiry = swab64(hr->hdec_expiry);
> +	hr->purr = swab64(hr->purr);
> +	hr->spurr = swab64(hr->spurr);
> +	hr->ic = swab64(hr->ic);
> +	hr->vtb = swab64(hr->vtb);
> +	hr->hdar = swab64(hr->hdar);
> +	hr->hdsisr = swab64(hr->hdsisr);
> +	hr->heir = swab64(hr->heir);
> +	hr->asdr = swab64(hr->asdr);
> +	hr->srr0 = swab64(hr->srr0);
> +	hr->srr1 = swab64(hr->srr1);
> +	hr->sprg[0] = swab64(hr->sprg[0]);
> +	hr->sprg[1] = swab64(hr->sprg[1]);
> +	hr->sprg[2] = swab64(hr->sprg[2]);
> +	hr->sprg[3] = swab64(hr->sprg[3]);
> +	hr->pidr = swab64(hr->pidr);
> +	hr->cfar = swab64(hr->cfar);
> +	hr->ppr = swab64(hr->ppr);
> +}
> +
>  static void save_hv_return_state(struct kvm_vcpu *vcpu, int trap,
>  				 struct hv_guest_state *hr)
>  {
> @@ -175,6 +217,8 @@ long kvmhv_enter_nested_guest(struct kvm_vcpu *vcpu)
>  				  sizeof(struct hv_guest_state));
>  	if (err)
>  		return H_PARAMETER;
> +	if (kvmppc_need_byteswap(vcpu))
> +		byteswap_hv_regs(&l2_hv);
>  	if (l2_hv.version != HV_GUEST_STATE_VERSION)
>  		return H_P2;
>  
> @@ -183,7 +227,8 @@ long kvmhv_enter_nested_guest(struct kvm_vcpu *vcpu)
>  				  sizeof(struct pt_regs));
>  	if (err)
>  		return H_PARAMETER;
> -
> +	if (kvmppc_need_byteswap(vcpu))
> +		byteswap_pt_regs(&l2_regs);
>  	if (l2_hv.vcpu_token >= NR_CPUS)
>  		return H_PARAMETER;
>  
> @@ -255,6 +300,10 @@ long kvmhv_enter_nested_guest(struct kvm_vcpu *vcpu)
>  	kvmhv_put_nested(l2);
>  
>  	/* copy l2_hv_state and regs back to guest */
> +	if (kvmppc_need_byteswap(vcpu)) {
> +		byteswap_hv_regs(&l2_hv);
> +		byteswap_pt_regs(&l2_regs);
> +	}
>  	err = kvm_vcpu_write_guest(vcpu, hv_ptr, &l2_hv,
>  				   sizeof(struct hv_guest_state));
>  	if (err)

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
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