Re: [PATCH 21/70] x86/boot/compressed/64: Add function to map a page unencrypted

Dave Hansen
2020-03-20T21:02:17Z

On 3/20/20 1:53 PM, David Rientjes wrote:
>> +
>> + /* Clear encryption flag and write new pte */
>> + pte = pte_clear_flags(*ptep, _PAGE_ENC);
>> + set_pte(ptep, pte);
>> +
>> + /* Flush TLB to map the page unencrypted */
>> + write_cr3(top_level_pgt);
>> +
> Is there a guarantee that this flushes the tlb if cr3 == top_level_pgt
> alrady without an invlpg?

Ahh, good catch.

It *never* flushes global pages. For a generic function like this, that
seems pretty dangerous because the PTEs it goes after could quite easily
be Global. It's also not _obviously_ correct if PCIDs are in play
(which I don't think they are on AMD).

A flush_tlb_global() is probably more appropriate. Better yet, is there
a reason not to use flush_tlb_kernel_range()? I don't think it's
necessary to whack the entire TLB for one PTE set.