[MODERATED] Re: [PATCH 8/8] L1TFv8 6

[Vlastimil Babka <vbabka@suse.cz>]

On 06/21/2018 11:02 AM, speck for Vlastimil Babka wrote:
> On 06/14/2018 12:48 AM, speck for Andi Kleen wrote:
>> +unsigned long max_swapfile_size(void)
>> +{
>> +	unsigned long pages;
>> +
>> +	pages = generic_max_swapfile_size();
>> +
>> +	if (boot_cpu_has_bug(X86_BUG_L1TF)) {
>> +		/* Limit the swap file size to MAX_PA/2 for L1TF workaround */
>> +		pages = min_t(unsigned long, l1tf_pfn_limit() + 1, pages);
> 
> Is this actually correct? IIUC l1tf_pfn_limit() is in page granularity,
> which are encoded in bits 12 to $LIMIT., but we have swap offsets in
> bits 9 to $LIMIT (after patch 2/8), i.e. 3 bits more? Same for the
> limits described in the changelog?

Yeah, I was able to verify this with some printk's, constructing a pte
with max allowed offset and printing it. In VM with 42bit limits, the
pte is 7ffffc000000000, so the unusable bits start with 38, not 41.

Also after more digging into this, I also suspect that the PAE case is
currently not mitigating. The pgtable-3level.h macros don't seem to flip
the bits. Also swap entries there use only the high pte word, whereas
most of the safe to use bits are in the low word.