linux-kernel - Re: [RFC Part1 PATCH 13/13] x86/kernel: add support to validate memory when changing C-bit

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Date:   Mon, 12 Apr 2021 15:05:42 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        ak@...ux.intel.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        "H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC Part1 PATCH 13/13] x86/kernel: add support to validate
 memory when changing C-bit

On Mon, Apr 12, 2021 at 07:55:01AM -0500, Brijesh Singh wrote:
> The cur_entry is updated by the hypervisor. While building the psc
> buffer the guest sets the cur_entry=0 and the end_entry point to the
> last valid entry. The cur_entry is incremented by the hypervisor after
> it successfully processes one 4K page. As per the spec, the hypervisor
> could get interrupted in middle of the page state change and cur_entry
> allows the guest to resume the page state change from the point where it
> was interrupted.

This is non-obvious and belongs in a comment above it. Otherwise it
looks weird.

> Since we can get interrupted while executing the PSC so just to be safe
> I re-initialized the scratch scratch area with our buffer instead of
> relying on old values.

Ditto.

> As per the spec the caller must check that the cur_entry > end_entry to
> determine whether all the entries are processed. If not then retry the
> state change. The hypervisor will skip the previously processed entries.
> The snp_page_state_vmgexit() is implemented to return only after all the
> entries are changed.

Ditto.

This whole mechanism of what the guest does and what the HV does, needs
to be explained in a bigger comment somewhere around there so that it is
clear what's going on.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette