Re: [RFC PATCH 1/1] x86/mm: Mark CoCo VM pages invalid while moving between private and shared

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Thu, 2023-07-06 at 09:41 -0700, Michael Kelley wrote:
> In a CoCo VM when a page transitions from private to shared, or vice
> versa, attributes in the PTE must be updated *and* the hypervisor
> must
> be notified of the change. Because there are two separate steps,
> there's
> a window where the settings are inconsistent.  Normally the code that
> initiates the transition (via set_memory_decrypted() or
> set_memory_encrypted()) ensures that the memory is not being accessed
> during a transition, so the window of inconsistency is not a problem.
> However, the load_unaligned_zeropad() function can read arbitrary
> memory
> pages at arbitrary times, which could access a transitioning page
> during
> the window.  In such a case, CoCo VM specific exceptions are taken
> (depending on the CoCo architecture in use).  Current code in those
> exception handlers recovers and does "fixup" on the result returned
> by
> load_unaligned_zeropad().  Unfortunately, this exception handling and
> fixup code is tricky and somewhat fragile.  At the moment, it is
> broken for both TDX and SEV-SNP.
> 
> There's also a problem with the current code in paravisor scenarios:
> TDX Partitioning and SEV-SNP in vTOM mode. The exceptions need
> to be forwarded from the paravisor to the Linux guest, but there
> are no architectural specs for how to do that.
> 
> To avoid these complexities of the CoCo exception handlers, change
> the core transition code in __set_memory_enc_pgtable() to do the
> following:
> 
> 1.  Remove aliasing mappings
> 2.  Remove the PRESENT bit from the PTEs of all transitioning pages
> 3.  Flush the TLB globally
> 4.  Flush the data cache if needed
> 5.  Set/clear the encryption attribute as appropriate
> 6.  Notify the hypervisor of the page status change
> 7.  Add back the PRESENT bit
> 
> With this approach, load_unaligned_zeropad() just takes its normal
> page-fault-based fixup path if it touches a page that is
> transitioning.
> As a result, load_unaligned_zeropad() and CoCo VM page transitioning
> are completely decoupled.  CoCo VM page transitions can proceed
> without needing to handle architecture-specific exceptions and fix
> things up. This decoupling reduces the complexity due to separate
> TDX and SEV-SNP fixup paths, and gives more freedom to revise and
> introduce new capabilities in future versions of the TDX and SEV-SNP
> architectures. Paravisor scenarios work properly without needing
> to forward exceptions.
> 
> This approach may make __set_memory_enc_pgtable() slightly slower
> because of touching the PTEs three times instead of just once. But
> the run time of this function is already dominated by the hypercall
> and the need to flush the TLB at least once and maybe twice. In any
> case, this function is only used for CoCo VM page transitions, and
> is already unsuitable for hot paths.

Excluding vm_unmap_aliases(), and just looking at the TLB flushes, it
kind of looks like this:
1. Clear present
2. TLB flush
3. Set C bit
4. Set Present bit
5. TLB flush

But if you instead did:
1. Clear Present and set C bit
2. TLB flush
3. Set Present bit (no flush)

Then you could still have only 1 TLB flush and 2 operations instead of
3. Otherwise it's the same load_unaligned_zeropad() benefits you are
looking for I think. But I'm not very educated on the private<->shared
conversion HW rules though, so maybe not.

> 
> The architecture specific callback function for notifying the
> hypervisor typically must translate guest kernel virtual addresses
> into guest physical addresses to pass to the hypervisor.  Because
> the PTEs are invalid at the time of callback, the code for doing the
> translation needs updating.  virt_to_phys() or equivalent continues
> to work for direct map addresses.  But vmalloc addresses cannot use
> vmalloc_to_page() because that function requires the leaf PTE to be
> valid. Instead, slow_virt_to_phys() must be used. Both functions
> manually walk the page table hierarchy, so performance is the same.

Just curious. Are vmalloc addresses supported here? It looks like in
SEV, but not TDX.