Re: Problem with pat_enable() and commit 72cbc8f04fe2

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.01.2023 06:59, Juergen Gross wrote:
> On 10.01.23 06:47, Juergen Gross wrote:
>> On 09.01.23 19:28, Michael Kelley (LINUX) wrote:
>>> I've come across a case with a VM running on Hyper-V that doesn't get
>>> MTRRs, but the PAT is functional.  (This is a Confidential VM using
>>> AMD's SEV-SNP encryption technology with the vTOM option.)  In this
>>> case, the changes in commit 72cbc8f04fe2 ("x86/PAT: Have pat_enabled()
>>> properly reflect state when running on Xen") apply.   pat_enabled() returns
>>> "true", but the MTRRs are not enabled.
>>>
>>> But with this commit, there's a problem.  Consider memremap() on a RAM
>>> region, called with MEMREMAP_WB plus MEMREMAP_DEC as the 3rd
>>> argument. Because of the request for a decrypted mapping,
>>> arch_memremap_can_ram_remap() returns false, and a new mapping
>>> must be created, which is appropriate.
>>>
>>> The following call stack results:
>>>
>>>    memremap()
>>>    arch_memremap_wb()
>>>    ioremap_cache()
>>>    __ioremap_caller()
>>>    memtype_reserve()  <--- pcm is _PAGE_CACHE_MODE_WB
>>>    pat_x_mtrr_type()  <-- only called after commit 72cbc8f04fe2
>>>
>>> pat_x_mtrr_type() returns _PAGE_CACHE_MODE_UC_MINUS because
>>> mtrr_type_lookup() fails.  As a result, memremap() erroneously creates the
>>> new mapping as uncached.   This uncached mapping is causing a significant
>>> performance problem in certain Hyper-V Confidential VM configurations.
>>>
>>> Any thoughts on resolving this?  Should memtype_reserve() be checking
>>> both pat_enabled() *and* whether MTRRs are enabled before calling
>>> pat_x_mtrr_type()?  Or does that defeat the purpose of commit
>>> 72cbc8f04fe2 in the Xen environment?
>>
>> I think pat_x_mtrr_type() should return _PAGE_CACHE_MODE_UC_MINUS only if
>> mtrr_type_lookup() is not failing and is returning a mode other than WB.

I agree.

> Another idea would be to let the mtrr_type_lookup() stub in
> arch/x86/include/asm/mtrr.h return MTRR_TYPE_WRBACK, enabling to simplify
> pud_set_huge() and pmd_set_huge() by removing the check for MTRR_TYPE_INVALID.

But that has a risk of ending up misleading: When there are no MTRRs, there
simply is no default type (in the absence of inspecting other criteria).

Jan