Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private memory during kexec() and reboot

[kirill.shutemov@xxxxxxxxxxxxxxx]

On Tue, Jun 13, 2023 at 12:51:23AM +0000, Huang, Kai wrote:
> On Mon, 2023-06-12 at 06:47 -0700, Dave Hansen wrote:
> > On 6/12/23 03:27, Huang, Kai wrote:
> > > So I think a __mb() after setting tdmr->pamt_4k_base should be good enough, as
> > > it guarantees when setting to any pamt_*_size happens, the valid pamt_4k_base
> > > will be seen by other cpus.
> > > 
> > > Does it make sense?
> > 
> > Just use a normal old atomic_t or set_bit()/test_bit().  They have
> > built-in memory barriers are are less likely to get botched.
> 
> Thanks for the suggestion.
> 
> Hi Dave, Kirill,
> 
> I'd like to check with you that whether we should introduce a mechanism to track
> TDX private pages for both this patch and the next.
> 
> As you can see this patch only deals PAMT pages due to couple of reasons that
> mnentioned in the changelog.  The next MCE patch handles all TDX private pages,
> but it uses SEAMCALL in the #MC handler.  Using SEAMCALL has two cons: 1) it is
> slow (probably doesn't matter, though); 2) it brings additional risk of
> triggering further #MC inside TDX module, although such risk should be a
> theoretical thing.
> 
> If we introduce a helper to mark a page as TDX private page, then both above
> patches can utilize it.  We don't need to consult TDMRs to get PAMT anymore in
> this patch (we will need a way to loop all TDX-usable memory pages, but this
> needs to be done anyway with TDX guests).  I believe eventually we can end up
> with less code.
> 
> In terms of how to do, for PAMT pages, we can set page->private to a TDX magic
> number because they come out of page allocator directly.  Secure-EPT pages are
> like PAMT pages too.  For TDX guest private pages, Sean is moving to implement
> KVM's own pseudo filesystem so they will have a unique mapping to identify.
> 
> https://github.com/sean-jc/linux/commit/40d338c8629287dda60a9f7c800ede8549295a7c
> 
> And my thinking is in this TDX host series, we can just handle PAMT pages.  Both
> secure-EPT and TDX guest private pages can be handled later in KVM TDX series. 
> I think eventually we can have a function like below to tell whether a page is
> TDX private page:
> 
> 	bool page_is_tdx_private(struct page *page)
> 	{
> 		if (page->private == TDX_PRIVATE_MAGIC)
> 			return true;
> 
> 		if (!page_mapping(page))
> 			return false;
> 
> 		return page_mapping(page)->a_ops == &kvm_gmem_ops;
> 	}
> 
> How does this sound?  Or any other comments?  Thanks!

If you going to take this path it has to be supported natively by
kvm_gmem_: it has to provide API for that. You should not assume that
page->private is free to use. It is owned by kvm_gmmem.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov