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

[Huang, Kai]

On Mon, 2022-11-28 at 10:26 +0100, David Hildenbrand wrote:
> On 28.11.22 10:21, Huang, Kai wrote:
> > 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.
> 
> Oh, so we're not also considering hotplugging memory to a TDX VM that 
> might not be backed by TDX. Got it.
> 
> So what you want to prevent is getting !TDX memory exposed to the buddy 
> such that it won't accidentally get allocated for a TDX guest, correct?

Yes correct.

> 
> In that case, memory notifiers would indeed be fine.
> 
> Thanks!
> 

Thanks.