linux-kernel - Re: [PATCH Part2 RFC v4 07/40] x86/sev: Split the physmap when adding the page in RMP table

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Date:   Thu, 15 Jul 2021 14:38:31 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>, tony.luck@...el.com,
        npmccallum@...hat.com, brijesh.ksingh@...il.com
Subject: Re: [PATCH Part2 RFC v4 07/40] x86/sev: Split the physmap when adding
 the page in RMP table



On 7/15/21 1:39 PM, Sean Christopherson wrote:
> On Thu, Jul 15, 2021, Brijesh Singh wrote:
>> The memfd_secrets uses the set_direct_map_{invalid,default}_noflush() and it
>> is designed to remove/add the present bit in the direct map. We can't use
>> them, because in our case the page may get accessed by the KVM (e.g
>> kvm_guest_write, kvm_guest_map etc).
> 
> But KVM should never access a guest private page, i.e. the direct map should
> always be restored to PRESENT before KVM attempts to access the page.
> 

Yes, KVM should *never* access the guest private pages. So, we could 
potentially enhance the RMPUPDATE() to check for the assigned and act 
accordingly.

Are you thinking something along the line of this:

int rmpupdate(struct page *page, struct rmpupdate *val)
{
	...
	
	/*
	 * If page is getting assigned in the RMP entry then unmap
	 * it from the direct map before its added in the RMP table.
	 */
	if (val.assigned)
		set_direct_map_invalid_noflush(page_to_virt(page), 1);

	...

	/*
	 * If the page is getting unassigned then restore the mapping
	 * in the direct map after its removed from the RMP table.
	 */
	if (!val.assigned)
		set_direct_map_default_noflush(page_to_virt(page), 1);
	
	...
}

thanks