netdev - Re: [PATCH] x86/sev-es: Expose __sev_es_ghcb_hv_call() to call ghcb hv call out of sev code

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Message-ID: <7bab8b73-e276-c23c-7a0a-2a6280e8a7d9@gmail.com>
Date:   Wed, 20 Oct 2021 23:09:03 +0800
From:   Tianyu Lan <ltykernel@...il.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>, kys@...rosoft.com,
        haiyangz@...rosoft.com, sthemmin@...rosoft.com, wei.liu@...nel.org,
        decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com,
        x86@...nel.org, hpa@...or.com, dave.hansen@...ux.intel.com,
        luto@...nel.org, peterz@...radead.org, davem@...emloft.net,
        kuba@...nel.org, gregkh@...uxfoundation.org, arnd@...db.de,
        jroedel@...e.de, brijesh.singh@....com, pgonda@...gle.com,
        akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
        rppt@...nel.org, tj@...nel.org, aneesh.kumar@...ux.ibm.com,
        saravanand@...com, hannes@...xchg.org, rientjes@...gle.com,
        michael.h.kelley@...rosoft.com, linux-arch@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, vkuznets@...hat.com,
        konrad.wilk@...cle.com, hch@....de, robin.murphy@....com,
        joro@...tes.org, parri.andrea@...il.com, dave.hansen@...el.com
Subject: Re: [PATCH] x86/sev-es: Expose __sev_es_ghcb_hv_call() to call ghcb
 hv call out of sev code

On 10/20/2021 10:39 PM, Borislav Petkov wrote:
> On Wed, Oct 20, 2021 at 10:23:06PM +0800, Tianyu Lan wrote:
>> This follows Joreg's previous comment and I implemented similar version in
>> the V! patchset([PATCH 05/13] HV: Add Write/Read MSR registers via ghcb page
>> https://lkml.org/lkml/2021/7/28/668).
>> "Instead, factor out a helper function which contains what Hyper-V needs and
>> use that in sev_es_ghcb_hv_call() and Hyper-V code."
>>
>> https://lkml.org/lkml/2021/8/2/375
> 
> If you wanna point to mails on a mailing list, you simply do
> 
> https://lore.kernel.org/r/<Message-id>
> 
> No need to use some random, unreliable web pages.

OK. Thanks for suggestion.

> 
> As to Joerg's suggestion, in the version I'm seeing, you're checking the
> *context* - and the one you sent today, avoids the __pa(ghcb) MSR write.
> 
> So which is it?
> 
> Because your current version will look at the context too, see
> 
> 	return verify_exception_info(ghcb, ctxt);
> 
> at the end of the function.
Both old and new patches are to avoid setting GHCB page address via MSR.
Paravisor is in charge of doing that and un-enlightened guest should not 
change it. The old one was in the patchset v1 "x86/Hyper-V: Add Hyper-V
Isolation VM support". The patch I sent today is based on your clean up 
patch and for review first. It should be in patchset "x86/Hyper-V: Add 
Hyper-V Isolation VM support."

> 
> So is the issue what Tom said that "the paravisor uses the same GHCB MSR
> and GHCB protocol, it just can't use __pa() to get the address of the
> GHCB."?

Yes, hyper-V enables vTOM in the CVM and GHCB page PA reported by 
paravisor contains vTOM bit. We need to use memremap() to map ghcb page 
before accessing GHCB page. __pa() doesn't work for PA with vTOM bit.
Otherwise, guest should not set GHCB page address and avoid conflict 
with paravisor.

> 
> If that is the case and the only thing you want is to avoid the GHCB PA
> write, then, in the future, we might drop that MSR write altogether on
> the enlightened Linux guests too and then the same function will be used
> by your paravisor and the Linux guest.

Yes, this is the target of the patch. Can we put the change in the 
Hyper-V patchset? Other patch has been fully reviewed.

Thanks.