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From:   "Kalra, Ashish" <Ashish.Kalra@amd.com>
To:     Peter Gonda <pgonda@google.com>
CC:     the arch/x86 maintainers <x86@kernel.org>,
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Subject: RE: [PATCH Part2 v6 35/49] KVM: SVM: Remove the long-lived GHCB host
 map
Thread-Topic: [PATCH Part2 v6 35/49] KVM: SVM: Remove the long-lived GHCB host
 map
Thread-Index: AQHYh9zZ2YXyQRnTe0iEslSS4jvfv61e+v3wgBRvg4CAAAHU4IABSe8AgAAAYBA=
Date:   Fri, 8 Jul 2022 15:59:33 +0000
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[AMD Official Use Only - General]

Hello Peter,

>> > I don't see anything mechanism for this patch to add the page state ch=
ange protection discussed. Can't another vCPU still convert the GHCB to pri=
vate?
>>
>> We do have the protections for GHCB getting mapped to private=20
>> specifically, there are new post_{map|unmap}_gfn functions added to veri=
fy if it is safe to map GHCB pages. There is a PSC spinlock added which pro=
tects again page state change for these mapped pages.
>> Below is the reference to this patch:
>> https://nam11.safelinks.protection.outlook.com/?url=3Dhttps%3A%2F%2Flore
>> .kernel.org%2Flkml%2Fcover.1655761627.git.ashish.kalra%40amd.com%2FT%2
>> F%23mafcaac7296eb9a92c0ea58730dbd3ca47a8e0756&amp;data=3D05%7C01%7CAshis
>> h.Kalra%40amd.com%7C647218cdb2a040bf354e08da60fa2968%7C3dd8961fe4884e6
>> 08e11a82d994e183d%7C0%7C0%7C637928924845082803%7CUnknown%7CTWFpbGZsb3d
>> 8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C
>> 3000%7C%7C%7C&amp;sdata=3Dss8%2F5qualccXQero9phARIG2wvYhtp8SMdve3GglZeU%
>> 3D&amp;reserved=3D0
>>
>> But do note that there is protection only for GHCB pages and there is=20
>> a need to add generic post_{map,unmap}_gfn() ops that can be used to ver=
ify that it's safe to map a given guest page in the hypervisor. This is a T=
ODO right now and probably this is something which UPM can address more cle=
anly.

>Thank you Ashish. I had missed that.

>Can you help me understand why its OK to use kvm_write_guest() for the
>|snp_certs_data| inside of snp_handle_ext_guest_request() in patch
>42/49? I would have thought we'd have the same 2M vs 4K mapping issues.

Preemption is not disabled there, hence the RMP page fault handler can do
the split of 2M to 4K on host pages without any issues.

Thanks,
Ashish