linux-kernel - RE: [PATCH Part2 v6 47/49] *fix for stale per-cpu pointer due to cond

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Message-ID: <SN6PR12MB27675A06ABFCAB1C999264D98EB49@SN6PR12MB2767.namprd12.prod.outlook.com>
Date:   Fri, 24 Jun 2022 16:44:14 +0000
From:   "Kalra, Ashish" <Ashish.Kalra@....com>
To:     Peter Gonda <pgonda@...gle.com>
CC:     the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
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        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
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        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
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Subject: RE: [PATCH Part2 v6 47/49] *fix for stale per-cpu pointer due to
 cond_resched during ghcb mapping

[AMD Official Use Only - General]

Hello Peter,
>>
>> From: Michael Roth <michael.roth@....com>
>>
>> Signed-off-by: Michael Roth <michael.roth@....com>

>Can you add a commit description here? Is this a fix for existing SEV-ES support or should this be incorporated into a patch in this series which adds this issue?

This actually fixes issues caused due to preemption happening in svm_prepare_switch_to_guest() when kvm_vcpu_map() is called to map in the GHCB before
entering the guest. 

This is a temporary fix and what we need to do is to prevent getting preempted after vcpu_enter_guest() has disabled preemption, have some ideas about
using gfn_to_pfn_cache() infrastructure to re-use the already mapped GHCB at guest exit, so that we can avoid calling kvm_vcpu_map() to re-map the 
GHCB.

Thanks,
Ashish

> ---
>  arch/x86/kvm/svm/svm.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 
> fced6ea423ad..f78e3b1bde0e 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1352,7 +1352,7 @@ static void svm_vcpu_free(struct kvm_vcpu *vcpu)  
> static void svm_prepare_switch_to_guest(struct kvm_vcpu *vcpu)  {
>         struct vcpu_svm *svm = to_svm(vcpu);
> -       struct svm_cpu_data *sd = per_cpu(svm_data, vcpu->cpu);
> +       struct svm_cpu_data *sd;
>
>         if (sev_es_guest(vcpu->kvm))
>                 sev_es_unmap_ghcb(svm); @@ -1360,6 +1360,10 @@ static 
> void svm_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
>         if (svm->guest_state_loaded)
>                 return;
>
> +       /* sev_es_unmap_ghcb() can resched, so grab per-cpu pointer afterward. */
> +       barrier();
> +       sd = per_cpu(svm_data, vcpu->cpu);
> +
>         /*
>          * Save additional host state that will be restored on VMEXIT (sev-es)
>          * or subsequent vmload of host save area.
> --
> 2.25.1
>