linux-kernel - Re: [PATCHv3 0/3] x86/tdx: Fix one more load_unaligned

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Date:   Thu, 6 Jul 2023 10:45:25 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>
Cc:     Dexuan Cui <decui@...rosoft.com>,
        "rick.p.edgecombe@...el.com" <rick.p.edgecombe@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv3 0/3] x86/tdx: Fix one more load_unaligned_zeropad()
 issue

On 7/6/23 09:48, Michael Kelley (LINUX) wrote:
> When a page is transitioning, the caller can and should ensure
> that it is not being accessed during the transition.  But we have
> the load_unaligned_zeropad() wildcard.   So do the following for
> the transition sequence in __set_memory_enc_pgtable():
> 
> 1.  Remove aliasing mappings
> 2.  Remove the PRESENT bit from the PTEs of all transitioning pages
> 3.  Flush the TLB globally
> 4.  Flush the data cache if needed
> 5.  Set/clear the encryption attribute as appropriate
> 6.  Notify the hypervisor of the page status change
> 7.  Add back the PRESENT bit
> 
> With this approach, load_unaligned_zeropad() just takes the
> normal page-fault-based fixup path if it touches a page that is
> transitioning.

Yes, this does seem much simpler.  It funnels everything through the
page fault handler and also doesn't require because careful about the
ordering of the private<=>shared conversions.