Re: [PATCH v2] KVM: SVM: improve the code readability for ASID management

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/08/21 20:09, Mingwei Zhang wrote:
> KVM SEV code uses bitmaps to manage ASID states. ASID 0 was always skipped
> because it is never used by VM. Thus, in existing code, ASID value and its
> bitmap postion always has an 'offset-by-1' relationship.
> 
> Both SEV and SEV-ES shares the ASID space, thus KVM uses a dynamic range
> [min_asid, max_asid] to handle SEV and SEV-ES ASIDs separately.
> 
> Existing code mixes the usage of ASID value and its bitmap position by
> using the same variable called 'min_asid'.
> 
> Fix the min_asid usage: ensure that its usage is consistent with its name;
> allocate extra size for ASID 0 to ensure that each ASID has the same value
> with its bitmap position. Add comments on ASID bitmap allocation to clarify
> the size change.
> 
> v1 -> v2:
>   - change ASID bitmap size to incorporate ASID 0 [sean]
>   - remove the 'fixes' line in commit message. [sean/joerg]
> 
> Signed-off-by: Mingwei Zhang <mizhang@google.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Cc: Marc Orr <marcorr@google.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Alper Gun <alpergun@google.com>
> Cc: Dionna Glaze <dionnaglaze@google.com>
> Cc: Sean Christopherson <seanjc@google.com>
> Cc: Vipin Sharma <vipinsh@google.com>
> Cc: Peter Gonda <pgonda@google.com>
> Cc: Joerg Roedel <joro@8bytes.org>
> ---
>   arch/x86/kvm/svm/sev.c | 36 +++++++++++++++++++++---------------
>   1 file changed, 21 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 8d36f0c73071..42d46c30f313 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -63,6 +63,7 @@ static DEFINE_MUTEX(sev_bitmap_lock);
>   unsigned int max_sev_asid;
>   static unsigned int min_sev_asid;
>   static unsigned long sev_me_mask;
> +static unsigned int nr_asids;
>   static unsigned long *sev_asid_bitmap;
>   static unsigned long *sev_reclaim_asid_bitmap;
>   
> @@ -77,11 +78,11 @@ struct enc_region {
>   /* 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;
> +	int ret, asid, error = 0;
>   
>   	/* Check if there are any ASIDs to reclaim before performing a flush */
> -	pos = find_next_bit(sev_reclaim_asid_bitmap, max_asid, min_asid);
> -	if (pos >= max_asid)
> +	asid = find_next_bit(sev_reclaim_asid_bitmap, nr_asids, min_asid);
> +	if (asid > max_asid)
>   		return -EBUSY;
>   
>   	/*
> @@ -114,15 +115,15 @@ static bool __sev_recycle_asids(int min_asid, int max_asid)
>   
>   	/* The flush process will flush all reclaimable SEV and SEV-ES ASIDs */
>   	bitmap_xor(sev_asid_bitmap, sev_asid_bitmap, sev_reclaim_asid_bitmap,
> -		   max_sev_asid);
> -	bitmap_zero(sev_reclaim_asid_bitmap, max_sev_asid);
> +		   nr_asids);
> +	bitmap_zero(sev_reclaim_asid_bitmap, nr_asids);
>   
>   	return true;
>   }
>   
>   static int sev_asid_new(struct kvm_sev_info *sev)
>   {
> -	int pos, min_asid, max_asid, ret;
> +	int asid, min_asid, max_asid, ret;
>   	bool retry = true;
>   	enum misc_res_type type;
>   
> @@ -142,11 +143,11 @@ static int sev_asid_new(struct kvm_sev_info *sev)
>   	 * SEV-enabled guests must use asid from min_sev_asid to max_sev_asid.
>   	 * SEV-ES-enabled guest can use from 1 to min_sev_asid - 1.
>   	 */
> -	min_asid = sev->es_active ? 0 : min_sev_asid - 1;
> +	min_asid = sev->es_active ? 1 : min_sev_asid;
>   	max_asid = sev->es_active ? min_sev_asid - 1 : max_sev_asid;
>   again:
> -	pos = find_next_zero_bit(sev_asid_bitmap, max_sev_asid, min_asid);
> -	if (pos >= max_asid) {
> +	asid = find_next_zero_bit(sev_asid_bitmap, max_sev_asid, min_asid);
> +	if (asid > max_asid) {
>   		if (retry && __sev_recycle_asids(min_asid, max_asid)) {
>   			retry = false;
>   			goto again;
> @@ -156,11 +157,11 @@ static int sev_asid_new(struct kvm_sev_info *sev)
>   		goto e_uncharge;
>   	}
>   
> -	__set_bit(pos, sev_asid_bitmap);
> +	__set_bit(asid, sev_asid_bitmap);
>   
>   	mutex_unlock(&sev_bitmap_lock);
>   
> -	return pos + 1;
> +	return asid;
>   e_uncharge:
>   	misc_cg_uncharge(type, sev->misc_cg, 1);
>   	put_misc_cg(sev->misc_cg);
> @@ -1854,12 +1855,17 @@ void __init sev_hardware_setup(void)
>   	min_sev_asid = edx;
>   	sev_me_mask = 1UL << (ebx & 0x3f);
>   
> -	/* Initialize SEV ASID bitmaps */
> -	sev_asid_bitmap = bitmap_zalloc(max_sev_asid, GFP_KERNEL);
> +	/*
> +	 * Initialize SEV ASID bitmaps. Allocate space for ASID 0 in the bitmap,
> +	 * even though it's never used, so that the bitmap is indexed by the
> +	 * actual ASID.
> +	 */
> +	nr_asids = max_sev_asid + 1;
> +	sev_asid_bitmap = bitmap_zalloc(nr_asids, GFP_KERNEL);
>   	if (!sev_asid_bitmap)
>   		goto out;
>   
> -	sev_reclaim_asid_bitmap = bitmap_zalloc(max_sev_asid, GFP_KERNEL);
> +	sev_reclaim_asid_bitmap = bitmap_zalloc(nr_asids, GFP_KERNEL);
>   	if (!sev_reclaim_asid_bitmap) {
>   		bitmap_free(sev_asid_bitmap);
>   		sev_asid_bitmap = NULL;
> @@ -1904,7 +1910,7 @@ void sev_hardware_teardown(void)
>   		return;
>   
>   	/* No need to take sev_bitmap_lock, all VMs have been destroyed. */
> -	sev_flush_asids(0, max_sev_asid);
> +	sev_flush_asids(1, max_sev_asid);
>   
>   	bitmap_free(sev_asid_bitmap);
>   	bitmap_free(sev_reclaim_asid_bitmap);
> 

Queued, thanks.

Paolo