linux-kernel - Re: [PATCH RFC v7 04/64] KVM: x86: Add 'fault_is

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Message-ID: <Y8Fr/F1RV0B8CHq5@zn.tnic>
Date:   Fri, 13 Jan 2023 15:34:36 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Michael Roth <michael.roth@....com>
Cc:     kvm@...r.kernel.org, linux-coco@...ts.linux.dev,
        linux-mm@...ck.org, linux-crypto@...r.kernel.org, x86@...nel.org,
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Subject: Re: [PATCH RFC v7 04/64] KVM: x86: Add 'fault_is_private' x86 op

On Wed, Jan 04, 2023 at 08:42:56PM -0600, Michael Roth wrote:
> Obviously I need to add some proper documentation for this, but a 1
> return basically means 'private_fault' pass-by-ref arg has been set
> with the appropriate value, whereas 0 means "there's no platform-specific
> handling for this, so if you have some generic way to determine this
> then use that instead".

Still binary, tho, and can be bool, right?

I.e., you can just as well do:

        if (static_call(kvm_x86_fault_is_private)(kvm, gpa, err, &private_fault))
                goto out;

at the call site.

> This is mainly to handle CONFIG_HAVE_KVM_PRIVATE_MEM_TESTING, which
> just parrots whatever kvm_mem_is_private() returns to support running
> KVM selftests without needed hardware/platform support. If we don't
> take care to skip this check where the above fault_is_private() hook
> returns 1, then it ends up breaking SNP in cases where the kernel has
> been compiled with CONFIG_HAVE_KVM_PRIVATE_MEM_TESTING, since SNP
> relies on the page fault flags to make this determination, not
> kvm_mem_is_private(), which normally only tracks the memory attributes
> set by userspace via KVM_SET_MEMORY_ATTRIBUTES ioctl.

Some of that explanation belongs into the commit message, which is a bit
lacking...

-- 
Regards/Gruss,
    Boris.

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