linux-kernel - Re: [PATCH v12 04/18] xen/pvh: Don't setup P2M tree.

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Date:	Fri, 3 Jan 2014 15:41:47 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	<linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
	<boris.ostrovsky@...cle.com>, <david.vrabel@...rix.com>,
	<stefano.stabellini@...citrix.com>, <mukesh.rathor@...cle.com>
Subject: Re: [PATCH v12 04/18] xen/pvh: Don't setup P2M tree.

On Tue, 31 Dec 2013, Konrad Rzeszutek Wilk wrote:
> P2M is not available for PVH. Fortunatly for us the
> P2M code already has mostly the support for auto-xlat guest thanks to
> commit 3d24bbd7dddbea54358a9795abaf051b0f18973c
> "grant-table: call set_phys_to_machine after mapping grant refs"
> which: "
> introduces set_phys_to_machine calls for auto_translated guests
> (even on x86) in gnttab_map_refs and gnttab_unmap_refs.
> translated by swiotlb-xen... " so we don't need to muck much.
> 
> with above mentioned "commit you'll get set_phys_to_machine calls
> from gnttab_map_refs and gnttab_unmap_refs but PVH guests won't do
> anything with them " (Stefano Stabellini) which is OK - we want
> them to be NOPs.
> 
> This is because we assume that an "IOMMU is always present on the
> plaform and Xen is going to make the appropriate IOMMU pagetable
> changes in the hypercall implementation of GNTTABOP_map_grant_ref
> and GNTTABOP_unmap_grant_ref, then eveything should be transparent
> from PVH priviligied point of view and DMA transfers involving
> foreign pages keep working with no issues[sp]
> 
> Otherwise we would need a P2M (and an M2P) for PVH priviligied to
> track these foreign pages .. (see arch/arm/xen/p2m.c)."
> (Stefano Stabellini).
> 
> We still have to inhibit the building of the P2M tree.
> That had been done in the past by not calling
> xen_build_dynamic_phys_to_machine (which setups the P2M tree
> and gives us virtual address to access them). But we are missing
> a check for xen_build_mfn_list_list - which was continuing to setup
> the P2M tree and would blow up at trying to get the virtual
> address of p2m_missing (which would have been setup by
> xen_build_dynamic_phys_to_machine).
> 
> Hence a check is needed to not call xen_build_mfn_list_list when
> running in auto-xlat mode.
> 
> Instead of replicating the check for auto-xlat in enlighten.c
> do it in the p2m.c code. The reason is that the xen_build_mfn_list_list
> is called also in xen_arch_post_suspend without any checks for
> auto-xlat. So for PVH or PV with auto-xlat - we would needlessly
> allocate space for an P2M tree.
> 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>

Acked-by: Stefano Stabellini <stefano.stabellini@...citrix.com>


>  arch/x86/xen/enlighten.c |  3 +--
>  arch/x86/xen/p2m.c       | 12 ++++++++++--
>  2 files changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> index 755e5bb..ab4dd70 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -1493,8 +1493,7 @@ asmlinkage void __init xen_start_kernel(void)
>  	x86_configure_nx();
>  
>  	/* Get mfn list */
> -	if (!xen_feature(XENFEAT_auto_translated_physmap))
> -		xen_build_dynamic_phys_to_machine();
> +	xen_build_dynamic_phys_to_machine();
>  
>  	/*
>  	 * Set up kernel GDT and segment registers, mainly so that
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index 2ae8699..fb7ee0a 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -280,6 +280,9 @@ void __ref xen_build_mfn_list_list(void)
>  {
>  	unsigned long pfn;
>  
> +	if (xen_feature(XENFEAT_auto_translated_physmap))
> +		return;
> +
>  	/* Pre-initialize p2m_top_mfn to be completely missing */
>  	if (p2m_top_mfn == NULL) {
>  		p2m_mid_missing_mfn = extend_brk(PAGE_SIZE, PAGE_SIZE);
> @@ -346,10 +349,15 @@ void xen_setup_mfn_list_list(void)
>  /* Set up p2m_top to point to the domain-builder provided p2m pages */
>  void __init xen_build_dynamic_phys_to_machine(void)
>  {
> -	unsigned long *mfn_list = (unsigned long *)xen_start_info->mfn_list;
> -	unsigned long max_pfn = min(MAX_DOMAIN_PAGES, xen_start_info->nr_pages);
> +	unsigned long *mfn_list;
> +	unsigned long max_pfn;
>  	unsigned long pfn;
>  
> +	 if (xen_feature(XENFEAT_auto_translated_physmap))
> +		return;
> +
> +	mfn_list = (unsigned long *)xen_start_info->mfn_list;
> +	max_pfn = min(MAX_DOMAIN_PAGES, xen_start_info->nr_pages);
>  	xen_max_p2m_pfn = max_pfn;
>  
>  	p2m_missing = extend_brk(PAGE_SIZE, PAGE_SIZE);
> -- 
> 1.8.3.1
> 
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