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

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Message-ID: <Ys2LDaKFE9+aoZKr@kernel.org>
Date:   Tue, 12 Jul 2022 17:54:05 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     "Kalra, Ashish" <Ashish.Kalra@....com>
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Subject: Re: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP
 fault for user address

On Tue, Jul 12, 2022 at 02:29:18PM +0000, Kalra, Ashish wrote:
> [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() ?

It's probably cpen coded for the sole reason that there is only one
call site, i.e. there has not been rational reason to have a helper
function.

Helpers are usually created only in-need basis, and since the need
comes from this patch set, it should include a patch, which simply
encapsulates it into a helper.

> 
> Thanks,
> Ashish

BR, Jarkko