linux-kernel - Re: [PATCH v5 15/15] KVM: SVM: Skip SEV cache flush if no ASIDs have been used

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Date:   Thu, 22 Apr 2021 09:30:02 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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>,
        Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v5 15/15] KVM: SVM: Skip SEV cache flush if no ASIDs have
 been used

On 22/04/21 04:11, Sean Christopherson wrote:
> +	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;

There's a tiny bug here which would cause sev_flush_asids to return 0
if there are reclaimed SEV ASIDs and the caller is looking for an SEV-ES
ASID, or vice versa.  The bug used to be in __sev_recycle_asids, you're
just moving the code.

It's not a big deal because sev_asid_new only retries once, but we might
as well fix it:

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 02b3426a9e39..403c6991e67c 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -74,12 +74,13 @@ struct enc_region {
  	unsigned long size;
  };
  
+/* Called with the sev_bitmap_lock held, or on shutdown  */
  static int sev_flush_asids(int min_asid, int max_asid)
  {
  	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);
+	pos = find_next_bit(sev_reclaim_asid_bitmap, max_asid, min_asid);
  	if (pos >= max_asid)
  		return -EBUSY;
  

Paolo

>   	/*
>   	 * DEACTIVATE will clear the WBINVD indicator causing DF_FLUSH to fail,
> @@ -87,14 +92,7 @@ static inline bool is_mirroring_enc_context(struct kvm *kvm)
>   /* 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 */
> @@ -1846,10 +1844,11 @@ 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)
>