Re: [PATCH 2/6] x86/pat: check for MTRRs enabled in memtype_reserve()

[Juergen Gross <jgross@suse.com>]

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On 07.02.23 09:49, Ingo Molnar wrote:
> 
> * Juergen Gross <jgross@suse.com> wrote:
> 
>> Today memtype_reserve() bails out early if pat_enabled() returns false.
>> The same can be done in case MTRRs aren't enabled.
>>
>> This will reinstate the behavior of memtype_reserve() before commit
>> 72cbc8f04fe2 ("x86/PAT: Have pat_enabled() properly reflect state when
>> running on Xen"). There have been reports about that commit breaking
>> SEV-SNP guests under Hyper-V, which was tried to be resolved by commit
>> 90b926e68f50 ("x86/pat: Fix pat_x_mtrr_type() for MTRR disabled case"),
>> but that again resulted in problems with Xen PV guests.
>>
>> Fixes: 72cbc8f04fe2 ("x86/PAT: Have pat_enabled() properly reflect state when running on Xen")
>> Fixes: 90b926e68f50 ("x86/pat: Fix pat_x_mtrr_type() for MTRR disabled case")
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>>   arch/x86/mm/pat/memtype.c | 10 +++++++---
>>   1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
>> index fb4b1b5e0dea..18f612b43763 100644
>> --- a/arch/x86/mm/pat/memtype.c
>> +++ b/arch/x86/mm/pat/memtype.c
>> @@ -557,8 +557,12 @@ int memtype_reserve(u64 start, u64 end, enum page_cache_mode req_type,
>>   		return -EINVAL;
>>   	}
>>   
>> -	if (!pat_enabled()) {
>> -		/* This is identical to page table setting without PAT */
>> +	/*
>> +	 * PAT disabled or MTRRs disabled don't require any memory type
>> +	 * tracking or type adjustments, as there can't be any conflicts
>> +	 * between PAT and MTRRs with at least one of both being disabled.
>> +	 */
>> +	if (!pat_enabled() || !mtrr_enabled()) {
>>   		if (new_type)
>>   			*new_type = req_type;
> 
> Doesn't memtype_reserve() also check for overlapping ranges & type
> compatibility in memtype_check_conflict(), etc., which can occur even in a
> pure PAT setup? Ie. are we 100% sure that in the !MTRR case it would be a
> NOP?
> 
> But even if it's a functional NOP as you claim, we'd still be better off if
> the memtype tree was still intact - instead of just turning off the API.

Yes, that's basically the issue discussed in [patch 0/6].

It should still be better than the original case (PAT and MTRR off, but
the ability to use PAT nevertheless), though.

> 
> Also, speling nit:
> 
>     s/one of both
>      /one or both

Hmm, but only if I drop the "at least". I don't really mind either way.


Juergen

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