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To:     Borislav Petkov <bp@alien8.de>
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Subject: RE: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP
 fault for user address
Thread-Topic: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP
 fault for user address
Thread-Index: AQHYrBDc30zm/ve2UUW13wSedLDd3a2nIMnggADB+gCAAMFX8IAieiDAgAC6pgCAAI/o0IAA2YMAgAAD/sCAACPGEA==
Date:   Sat, 3 Sep 2022 06:57:51 +0000
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[AMD Official Use Only - General]

So essentially we want to map the faulting address to a RMP entry, consider=
ing the fact that a 2M host hugepage can be mapped as=20
4K RMP table entries and 1G host hugepage can be mapped as 2M RMP table ent=
ries.

Hence, this mask computation is done as:
mask =3D pages_per_hpage(level) - pages_per_hpage(level -1);

and the final faulting pfn is computed as:
pfn |=3D (address >> PAGE_SHIFT) & mask;
     =20
Thanks,
Ashish   =20

-----Original Message-----
From: Kalra, Ashish=20
Sent: Saturday, September 3, 2022 12:51 AM
To: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org; linux-kernel@vger.kernel.org; kvm@vger.kernel.org; linu=
x-coco@lists.linux.dev; linux-mm@kvack.org; linux-crypto@vger.kernel.org; t=
glx@linutronix.de; mingo@redhat.com; jroedel@suse.de; Lendacky, Thomas <Tho=
mas.Lendacky@amd.com>; hpa@zytor.com; ardb@kernel.org; pbonzini@redhat.com;=
 seanjc@google.com; vkuznets@redhat.com; jmattson@google.com; luto@kernel.o=
rg; dave.hansen@linux.intel.com; slp@redhat.com; pgonda@google.com; peterz@=
infradead.org; srinivas.pandruvada@linux.intel.com; rientjes@google.com; do=
vmurik@linux.ibm.com; tobin@ibm.com; Roth, Michael <Michael.Roth@amd.com>; =
vbabka@suse.cz; kirill@shutemov.name; ak@linux.intel.com; tony.luck@intel.c=
om; marcorr@google.com; sathyanarayanan.kuppuswamy@linux.intel.com; alpergu=
n@google.com; dgilbert@redhat.com; jarkko@kernel.org
Subject: RE: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RM=
P fault for user address

[AMD Official Use Only - General]

Hello Boris,

>> Yes we want to map the faulting address to a RMP entry, but hugepage=20
>> entries in RMP table are basically subpage 4K entries. So it is a 4K=20
>> entry when the page is a 2M one and also a 4K entry when the page is=20
>> a 1G one.

>Wait, what?!

>APM v2 section "15.36.11 Large Page Management" and PSMASH are then for wh=
at exactly?

This is what exactly PSMASH is for, in case the 2MB RMP entry needs to be s=
mashed if guest PVALIDATES a 4K page, the HV will need to PSMASH the 2MB RM=
P entry to corresponding 4K RMP entries during #VMEXIT(NPF).

What I meant above is that 4K RMP table entries need to be available in cas=
e the 2MB RMP entry needs to be smashed.=20

>> That's why the computation to get a 4K page index within a 2M/1G=20
>> hugepage mapping is required.

>What if a guest RMP-faults on a 2M page and there's a corresponding 2M RMP=
 entry? What do you need the 4K entry then for?

There is no fault here, if guest pvalidates a 2M page that is backed by a 2=
MB RMP entry.
We need the 4K entries in case the guest pvalidates a 4K page that is mappe=
d by a 2MB RMP entry.

>Hell, __snp_lookup_rmpentry() even tries to return the proper page level..=
.

>/me looks in disbelief in your direction...

Thanks,
Ashish