linux-kernel - Re: [PATCH v7 10/20] x86/virt/tdx: Use all system memory when initializing TDX module as TDX memory

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Message-ID: <8300f1098aa8fbfae711313be41ee44cb1203d62.camel@intel.com>
Date:   Mon, 28 Nov 2022 09:21:05 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "Williams, Dan J" <dan.j.williams@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "david@...hat.com" <david@...hat.com>
CC:     "Hansen, Dave" <dave.hansen@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        "Brown, Len" <len.brown@...el.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>
Subject: Re: [PATCH v7 10/20] x86/virt/tdx: Use all system memory when
 initializing TDX module as TDX memory

On Mon, 2022-11-28 at 09:43 +0100, David Hildenbrand wrote:
> On 28.11.22 09:38, Huang, Kai wrote:
> > On Fri, 2022-11-25 at 10:28 +0100, David Hildenbrand wrote:
> > > On 24.11.22 10:06, Huang, Kai wrote:
> > > > On Wed, 2022-11-23 at 17:50 -0800, Dan Williams wrote:
> > > > > >     
> > > > > > @@ -968,6 +969,15 @@ int arch_add_memory(int nid, u64 start, u64 size,
> > > > > >     	unsigned long start_pfn = start >> PAGE_SHIFT;
> > > > > >     	unsigned long nr_pages = size >> PAGE_SHIFT;
> > > > > >     
> > > > > > +	/*
> > > > > > +	 * For now if TDX is enabled, all pages in the page allocator
> > > > > > +	 * must be TDX memory, which is a fixed set of memory regions
> > > > > > +	 * that are passed to the TDX module.  Reject the new region
> > > > > > +	 * if it is not TDX memory to guarantee above is true.
> > > > > > +	 */
> > > > > > +	if (!tdx_cc_memory_compatible(start_pfn, start_pfn + nr_pages))
> > > > > > +		return -EINVAL;
> > > > > 
> > > > > arch_add_memory() does not add memory to the page allocator.  For
> > > > > example, memremap_pages() uses arch_add_memory() and explicitly does not
> > > > > release the memory to the page allocator.
> > > > 
> > > > Indeed.  Sorry I missed this.
> > > > 
> > > > > This check belongs in
> > > > > add_memory_resource() to prevent new memory that violates TDX from being
> > > > > onlined.
> > > > 
> > > > This would require adding another 'arch_cc_memory_compatible()' to the common
> > > > add_memory_resource() (I actually long time ago had such patch to work with the
> > > > memremap_pages() you mentioned above).
> > > > 
> > > > How about adding a memory_notifier to the TDX code, and reject online of TDX
> > > > incompatible memory (something like below)?  The benefit is this is TDX code
> > > > self contained and won't pollute the common mm code:
> > > > 
> > > > +static int tdx_memory_notifier(struct notifier_block *nb,
> > > > +                              unsigned long action, void *v)
> > > > +{
> > > > +       struct memory_notify *mn = v;
> > > > +
> > > > +       if (action != MEM_GOING_ONLINE)
> > > > +               return NOTIFY_OK;
> > > > +
> > > > +       /*
> > > > +        * Not all memory is compatible with TDX.  Reject
> > > > +        * online of any incompatible memory.
> > > > +        */
> > > > +       return tdx_cc_memory_compatible(mn->start_pfn,
> > > > +                       mn->start_pfn + mn->nr_pages) ? NOTIFY_OK : NOTIFY_BAD;
> > > > +}
> > > > +
> > > > +static struct notifier_block tdx_memory_nb = {
> > > > +       .notifier_call = tdx_memory_notifier,
> > > > +};
> > > 
> > > With mhp_memmap_on_memory() some memory might already be touched during
> > > add_memory() (because part of the hotplug memory is used for holding the
> > > memmap), not when actually onlining memory. So in that case, this would
> > > be too late.
> > 
> > Hi David,
> > 
> > Thanks for the review!
> > 
> > Right. The memmap pages are added to the zone before online_pages(), but IIUC
> > those memmap pages will never be free pages thus won't be allocated by the page
> > allocator, correct?  Therefore in practice there won't be problem even they are
> > not TDX compatible memory.
> 
> But that memory will be read/written. Isn't that an issue, for example, 
> if memory doesn't get accepted etc?
> 

Sorry I don't quite understand "if memory doesn't get accepted" mean.  Do you
mean accepted by the TDX module?

Only the host kernel will read/write those memmap pages.  The TDX module won't
(as they won't be allocated to be used as TDX guest memory or TDX module
metadata).  So it's fine.