lkml.org 
[lkml]   [2023]   [Oct]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH 10/13] x86/tdx: Convert shared memory back to private on kexec
From
On 10/5/2023 4:28 PM, Kirill A. Shutemov wrote:
> On Thu, Oct 05, 2023 at 01:41:38PM -0500, Kalra, Ashish wrote:
>>> +static void unshare_all_memory(bool unmap)
>>> +{
>>> + unsigned long addr, end;
>>> + long found = 0, shared;
>>> +
>>> + /*
>>> + * Walk direct mapping and convert all shared memory back to private,
>>> + */
>>> +
>>> + addr = PAGE_OFFSET;
>>> + end = PAGE_OFFSET + get_max_mapped();
>>> +
>>> + while (addr < end) {
>>> + unsigned long size;
>>> + unsigned int level;
>>> + pte_t *pte;
>>> +
>>> + pte = lookup_address(addr, &level);
>>
>> IIRC, you were earlier walking the direct mapping using
>> walk_page_range_novma(), any particular reason to use lookup_address()
>> instead ?
>
> walk_page_range_novma() wants mmap lock to be taken, but it is tricky as
> we run here from atomic context in case of crash.
>
> I considered using trylock to bypass the limitation, but it is a hack.
>
>>
>>> + size = page_level_size(level);
>>> +
>>> + if (pte && pte_decrypted(*pte)) {
>>
>> Additionally need to add check for pte_none() here to handle physical memory
>> holes in direct mapping.
>
> lookup_address() returns NULL for none entries.
>

Looking at lookup_address_in_pgd(), at pte level it is simply returning
pte_offset_kernel() and there does not seem to be a check for returning
NULL if pte_none() ?

>>> + int pages = size / PAGE_SIZE;
>>> +
>>> + /*
>>> + * Touching memory with shared bit set triggers implicit
>>> + * conversion to shared.
>>> + *
>>> + * Make sure nobody touches the shared range from
>>> + * now on.
>>> + *
>>> + * Bypass unmapping for crash scenario. Unmapping
>>> + * requires sleepable context, but in crash case kernel
>>> + * hits the code path with interrupts disabled.
>>
>> In case of SNP we will need to temporarily enable interrupts during this
>> unsharing as we invoke set_memory_encrypted() which then hits a BUG_ON() in
>> cpa_flush() if interrupts are disabled.
>
> Do you really need full set_memory_encrypted()? Can't you do something
> ligher?
>
We need to modify the PTE for setting c-bit to 1 so that will require
cpa_flush(), though probably can add something lighter to do
clflush_cache_range() directly ?

Thanks,
Ashish

\
 
 \ /
  Last update: 2023-10-06 00:02    [W:0.168 / U:2.788 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site