linux-kernel - RE: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP fault for user address

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Date:   Tue, 12 Jul 2022 14:29:18 +0000
From:   "Kalra, Ashish" <Ashish.Kalra@....com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
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Subject: RE: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP
 fault for user address

[AMD Official Use Only - General]

>> +static int handle_user_rmp_page_fault(struct pt_regs *regs, unsigned long error_code,
>> +				      unsigned long address)
>> +{
>> +	int rmp_level, level;
>> +	pte_t *pte;
>> +	u64 pfn;
>> +
>> +	pte = lookup_address_in_mm(current->mm, address, &level);

>As discussed in [1], the lookup should be done in kvm->mm, along the lines of host_pfn_mapping_level().

With lookup_address_in_mm() now removed in 5.19, this is now using lookup_address_in_pgd() though still using non init-mm, and as mentioned here in [1], it makes sense to
not use lookup_address_in_pgd() as it does not play nice with userspace mappings, e.g. doesn't disable IRQs to block TLB shootdowns and doesn't use READ_ONCE()
to ensure an upper level entry isn't converted to a huge page between checking the PAGE_SIZE bit and grabbing the address of the next level down.

But is KVM going to provide its own variant of lookup_address_in_pgd() that is safe for use with user addresses, i.e., a generic version of lookup_address() on kvm->mm or we need to
duplicate page table walking code of host_pfn_mapping_level() ?

Thanks,
Ashish

>[1] https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fkvm%2FYmwIi3bXr%252F1yhYV%252F%40google.com%2F&amp;data=05%7C01%7CAshish.Kalra%40amd.com%>7Ce300014162fc4d8b452708da63fdb970%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637932238689925974%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%>7C%7C%7C&amp;sdata=GxPrEUxuVNEm6COdfHCILOwp9yuX48gpoYmtrOwMx8Q%3D&amp;reserved=0