[PATCHv9 10/17] x86/tdx: Convert shared memory back to private on kexec

[Huang, Kai]

On Mon, 2024-03-25 at 12:39 +0200, Kirill A. Shutemov wrote:
> TDX guests allocate shared buffers to perform I/O. It is done by
> allocating pages normally from the buddy allocator and converting them
> to shared with set_memory_decrypted().
> 
> The second kernel has no idea what memory is converted this way. It only
> sees E820_TYPE_RAM.
> 
> Accessing shared memory via private mapping is fatal. It leads to
> unrecoverable TD exit.
> 
> On kexec walk direct mapping and convert all shared memory back to
> private. It makes all RAM private again and second kernel may use it
> normally.
> 
> The conversion occurs in two steps: stopping new conversions and
> unsharing all memory. In the case of normal kexec, the stopping of
> conversions takes place while scheduling is still functioning. This
> allows for waiting until any ongoing conversions are finished. The
> second step is carried out when all CPUs except one are inactive and
> interrupts are disabled. This prevents any conflicts with code that may
> access shared memory.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
> Reviewed-by: Rick Edgecombe <rick.p.edgecombe at intel.com>
> 

Reviewed-by: Kai Huang <kai.huang at intel.com>