linux-kernel - Re: [PATCH v3 13/13] KVM: SVM: Skip SEV cache flush if no ASIDs have been used

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Message-ID: <7ba2f3a7-2e8e-39fa-dbf6-b2781f244486@amd.com>
Date:   Fri, 22 Jan 2021 15:16:37 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Borislav Petkov <bp@...e.de>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v3 13/13] KVM: SVM: Skip SEV cache flush if no ASIDs have
 been used

On 1/22/21 2:21 PM, Sean Christopherson wrote:
> Skip SEV's expensive WBINVD and DF_FLUSH if there are no SEV ASIDs
> waiting to be reclaimed, e.g. if SEV was never used.  This "fixes" an
> issue where the DF_FLUSH fails during hardware teardown if the original
> SEV_INIT failed.  Ideally, SEV wouldn't be marked as enabled in KVM if
> SEV_INIT fails, but that's a problem for another day.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Tom Lendacky <thomas.lendacky@....com>

> ---
>   arch/x86/kvm/svm/sev.c | 23 +++++++++++------------
>   1 file changed, 11 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 73da2af1e25d..0a4715e60b88 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -56,9 +56,14 @@ struct enc_region {
>   	unsigned long size;
>   };
>   
> -static int sev_flush_asids(void)
> +static int sev_flush_asids(int min_asid, int max_asid)
>   {
> -	int ret, error = 0;
> +	int ret, pos, error = 0;
> +
> +	/* Check if there are any ASIDs to reclaim before performing a flush */
> +	pos = find_next_bit(sev_reclaim_asid_bitmap, max_sev_asid, min_asid);
> +	if (pos >= max_asid)
> +		return -EBUSY;
>   
>   	/*
>   	 * DEACTIVATE will clear the WBINVD indicator causing DF_FLUSH to fail,
> @@ -80,14 +85,7 @@ static int sev_flush_asids(void)
>   /* Must be called with the sev_bitmap_lock held */
>   static bool __sev_recycle_asids(int min_asid, int max_asid)
>   {
> -	int pos;
> -
> -	/* Check if there are any ASIDs to reclaim before performing a flush */
> -	pos = find_next_bit(sev_reclaim_asid_bitmap, max_sev_asid, min_asid);
> -	if (pos >= max_asid)
> -		return false;
> -
> -	if (sev_flush_asids())
> +	if (sev_flush_asids(min_asid, max_asid))
>   		return false;
>   
>   	/* The flush process will flush all reclaimable SEV and SEV-ES ASIDs */
> @@ -1324,10 +1322,11 @@ void sev_hardware_teardown(void)
>   	if (!sev_enabled)
>   		return;
>   
> +	/* No need to take sev_bitmap_lock, all VMs have been destroyed. */
> +	sev_flush_asids(0, max_sev_asid);
> +
>   	bitmap_free(sev_asid_bitmap);
>   	bitmap_free(sev_reclaim_asid_bitmap);
> -
> -	sev_flush_asids();
>   }
>   
>   int sev_cpu_init(struct svm_cpu_data *sd)
>