linux-kernel - Re: [PATCH Part2 RFC v4 25/40] KVM: SVM: Reclaim the guest pages when SEV-SNP VM terminates

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Message-ID: <YPXPB1WMW8WY9cZ7@google.com>
Date:   Mon, 19 Jul 2021 19:14:15 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     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 25/40] KVM: SVM: Reclaim the guest pages
 when SEV-SNP VM terminates

On Mon, Jul 19, 2021, Sean Christopherson wrote:
> On Mon, Jul 19, 2021, Brijesh Singh wrote:
> > 
> > On 7/19/21 12:18 PM, Sean Christopherson wrote:
> > > > 
> > > > Okay, I will add helper to make things easier. One case where we will
> > > > need to directly call the rmpupdate() is during the LAUNCH_UPDATE
> > > > command. In that case the page is private and its immutable bit is also
> > > > set. This is because the firmware makes change to the page, and we are
> > > > required to set the immutable bit before the call.
> > > 
> > > Or do "int rmp_make_firmware(u64 pfn, bool immutable)"?
> > 
> > That's not what we need.
> > 
> > We need 'rmp_make_private() + immutable' all in one RMPUPDATE.  Here is the
> > snippet from SNP_LAUNCH_UPDATE.
> 
> Ah, not firmwrare, gotcha.  But we can still use a helper, e.g. an inner
> double-underscore helper, __rmp_make_private().

Hmm, looking at it again, I think I also got confused by the comment for the VMSA
page:

	/* Transition the VMSA page to a firmware state. */
 	e.assigned = 1;
	e.immutable = 1;
	e.asid = sev->asid;
	e.gpa = -1;
	e.pagesize = RMP_PG_SIZE_4K;

Unlike __snp_alloc_firmware_pages() in the CCP code, the VMSA is associated with
the guest's ASID, just not a GPA.  I.e. the VMSA is more of a specialized guest
private page, as opposed to a dedicated firmware page.  I.e. a __rmp_make_private()
and/or rmp_make_private_immutable() definitely seems like a good idea.