linux-kernel - Re: Problem with pat_enable() and commit 72cbc8f04fe2

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Date:   Tue, 10 Jan 2023 11:01:03 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: Problem with pat_enable() and commit 72cbc8f04fe2

On 10.01.23 10:38, Jan Beulich wrote:
> 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).

I've sent a patch checking for MTRR_TYPE_INVALID in pat_x_mtrr_type(). This
seemed to be a less intrusive change.

The idea to modify the stub came up as a result of looking at mtrr_type_lookup()
use cases after writing my patch. All users now take an action if the returned
type is not WB and not INVALID. So it would be a modification tailored for
today's mtrr_type_lookup() users only.


Juergen

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