Re: [PATCH v8 3/8] kvmppc: Shared pages support for secure guests

[Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>]

> A secure guest will share some of its pages with hypervisor (Eg. virtio
> bounce buffers etc). Support sharing of pages between hypervisor and
> ultravisor.

A brief note about what a shared page is would help (a page belonging
to the SVM but in normal memory and with decrypted contents)? Either
here or in the function header of kvmppc_h_svm_page_in() where we
handle shared pages.

> 
> Once a secure page is converted to shared page, the device page is

Maybe useful to add "the device page (representing the secure page") is ...

> unmapped from the HV side page tables.
> 
> Signed-off-by: Bharata B Rao <bharata@linux.ibm.com>
> ---
>  arch/powerpc/include/asm/hvcall.h  |  3 ++
>  arch/powerpc/kvm/book3s_hv_uvmem.c | 65 ++++++++++++++++++++++++++++--
>  2 files changed, 65 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/hvcall.h b/arch/powerpc/include/asm/hvcall.h
> index 2595d0144958..4e98dd992bd1 100644
> --- a/arch/powerpc/include/asm/hvcall.h
> +++ b/arch/powerpc/include/asm/hvcall.h
> @@ -342,6 +342,9 @@
>  #define H_TLB_INVALIDATE	0xF808
>  #define H_COPY_TOFROM_GUEST	0xF80C
> 
> +/* Flags for H_SVM_PAGE_IN */
> +#define H_PAGE_IN_SHARED        0x1
> +
>  /* Platform-specific hcalls used by the Ultravisor */
>  #define H_SVM_PAGE_IN		0xEF00
>  #define H_SVM_PAGE_OUT		0xEF04
> diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> index a1eccb065ba9..bcecb643a730 100644
> --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> @@ -46,6 +46,7 @@ struct kvmppc_uvmem_page_pvt {
>  	unsigned long *rmap;
>  	unsigned int lpid;
>  	unsigned long gpa;
> +	bool skip_page_out;
>  };
> 
>  /*
> @@ -159,6 +160,53 @@ kvmppc_svm_page_in(struct vm_area_struct *vma, unsigned long start,
>  	return ret;
>  }
> 
> +/*
> + * Shares the page with HV, thus making it a normal page.
> + *
> + * - If the page is already secure, then provision a new page and share
> + * - If the page is a normal page, share the existing page
> + *
> + * In the former case, uses the dev_pagemap_ops migrate_to_ram handler
> + * to unmap the device page from QEMU's page tables.
> + */
> +static unsigned long
> +kvmppc_share_page(struct kvm *kvm, unsigned long gpa, unsigned long page_shift)
> +{
> +
> +	int ret = H_PARAMETER;
> +	struct page *uvmem_page;
> +	struct kvmppc_uvmem_page_pvt *pvt;
> +	unsigned long pfn;
> +	unsigned long *rmap;
> +	struct kvm_memory_slot *slot;
> +	unsigned long gfn = gpa >> page_shift;
> +	int srcu_idx;
> +
> +	srcu_idx = srcu_read_lock(&kvm->srcu);
> +	slot = gfn_to_memslot(kvm, gfn);
> +	if (!slot)
> +		goto out;
> +
> +	rmap = &slot->arch.rmap[gfn - slot->base_gfn];
> +	if (kvmppc_rmap_type(rmap) == KVMPPC_RMAP_UVMEM_PFN) {
> +		uvmem_page = pfn_to_page(*rmap & ~KVMPPC_RMAP_UVMEM_PFN);
> +		pvt = (struct kvmppc_uvmem_page_pvt *)
> +			uvmem_page->zone_device_data;
> +		pvt->skip_page_out = true;
> +	}
> +
> +	pfn = gfn_to_pfn(kvm, gfn);
> +	if (is_error_noslot_pfn(pfn))
> +		goto out;
> +
> +	if (!uv_page_in(kvm->arch.lpid, pfn << page_shift, gpa, 0, page_shift))
> +		ret = H_SUCCESS;
> +	kvm_release_pfn_clean(pfn);
> +out:
> +	srcu_read_unlock(&kvm->srcu, srcu_idx);
> +	return ret;
> +}
> +
>  /*
>   * H_SVM_PAGE_IN: Move page from normal memory to secure memory.

Would help to mention/remind here what a shared page is.

>   */
> @@ -177,9 +225,12 @@ kvmppc_h_svm_page_in(struct kvm *kvm, unsigned long gpa,
>  	if (page_shift != PAGE_SHIFT)
>  		return H_P3;
> 
> -	if (flags)
> +	if (flags & ~H_PAGE_IN_SHARED)
>  		return H_P2;
> 
> +	if (flags & H_PAGE_IN_SHARED)
> +		return kvmppc_share_page(kvm, gpa, page_shift);
> +
>  	ret = H_PARAMETER;
>  	srcu_idx = srcu_read_lock(&kvm->srcu);
>  	down_read(&kvm->mm->mmap_sem);
> @@ -252,8 +303,16 @@ kvmppc_svm_page_out(struct vm_area_struct *vma, unsigned long start,
>  	pvt = spage->zone_device_data;
>  	pfn = page_to_pfn(dpage);
> 
> -	ret = uv_page_out(pvt->lpid, pfn << page_shift, pvt->gpa, 0,
> -			  page_shift);
> +	/*
> +	 * This function is used in two cases:
> +	 * - When HV touches a secure page, for which we do UV_PAGE_OUT
> +	 * - When a secure page is converted to shared page, we touch
> +	 *   the page to essentially unmap the device page. In this
> +	 *   case we skip page-out.
> +	 */
> +	if (!pvt->skip_page_out)
> +		ret = uv_page_out(pvt->lpid, pfn << page_shift, pvt->gpa, 0,
> +				  page_shift);
> 
>  	if (ret == U_SUCCESS)
>  		*mig.dst = migrate_pfn(pfn) | MIGRATE_PFN_LOCKED;
> -- 
> 2.21.0