Re: [PATCH 2/8] KVM: x86/xen: Introduce kvm_xen_has_64bit_shinfo() macro

[David Laight <david.laight.linux@gmail.com>]

On Fri,  5 Jun 2026 15:17:27 +0100
David Woodhouse <dwmw2@infradead.org> wrote:

> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> Add a kvm_xen_has_64bit_shinfo() helper macro to replace the repeated
> pattern of 'IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode'
> throughout the Xen emulation code. The macro uses READ_ONCE() to
> ensure a consistent snapshot of the flag, which can be changed by
> another vCPU at any time.

If another vCPU changes the flag then isn't it all broken?
The code seems to need the value to be stable.

-- David

> 
> This is the KVM equivalent of Xen's !has_32bit_shinfo().
> 
> Assisted-by: Kiro:claude-opus-4.6-1m
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>  arch/x86/kvm/xen.c | 16 ++++++++--------
>  arch/x86/kvm/xen.h |  5 +++++
>  2 files changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index a7ab19f38b59..acd3cd87dd2f 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -72,7 +72,7 @@ static int kvm_xen_shared_info_init(struct kvm *kvm)
>  	BUILD_BUG_ON(offsetof(struct shared_info, wc) != 0xc00);
>  	BUILD_BUG_ON(offsetof(struct shared_info, wc_sec_hi) != 0xc0c);
>  
> -	if (IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode) {
> +	if (kvm_xen_has_64bit_shinfo(kvm)) {
>  		struct shared_info *shinfo = gpc->khva;
>  
>  		wc_sec_hi = &shinfo->wc_sec_hi;
> @@ -390,7 +390,7 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
>  	BUILD_BUG_ON(sizeof_field(struct vcpu_runstate_info, time) !=
>  		     sizeof(vx->runstate_times));
>  
> -	if (IS_ENABLED(CONFIG_64BIT) && v->kvm->arch.xen.long_mode) {
> +	if (kvm_xen_has_64bit_shinfo(v->kvm)) {
>  		user_len = sizeof(struct vcpu_runstate_info);
>  		times_ofs = offsetof(struct vcpu_runstate_info,
>  				     state_entry_time);
> @@ -661,7 +661,7 @@ void kvm_xen_inject_pending_events(struct kvm_vcpu *v)
>  	}
>  
>  	/* Now gpc->khva is a valid kernel address for the vcpu_info */
> -	if (IS_ENABLED(CONFIG_64BIT) && v->kvm->arch.xen.long_mode) {
> +	if (kvm_xen_has_64bit_shinfo(v->kvm)) {
>  		struct vcpu_info *vi = gpc->khva;
>  
>  		asm volatile(LOCK_PREFIX "orq %0, %1\n"
> @@ -978,7 +978,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
>  		 * address, that's actually OK. kvm_xen_update_runstate_guest()
>  		 * will cope.
>  		 */
> -		if (IS_ENABLED(CONFIG_64BIT) && vcpu->kvm->arch.xen.long_mode)
> +		if (kvm_xen_has_64bit_shinfo(vcpu->kvm))
>  			sz = sizeof(struct vcpu_runstate_info);
>  		else
>  			sz = sizeof(struct compat_vcpu_runstate_info);
> @@ -1424,7 +1424,7 @@ static int kvm_xen_hypercall_complete_userspace(struct kvm_vcpu *vcpu)
>  
>  static inline int max_evtchn_port(struct kvm *kvm)
>  {
> -	if (IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode)
> +	if (kvm_xen_has_64bit_shinfo(kvm))
>  		return EVTCHN_2L_NR_CHANNELS;
>  	else
>  		return COMPAT_EVTCHN_2L_NR_CHANNELS;
> @@ -1446,7 +1446,7 @@ static bool wait_pending_event(struct kvm_vcpu *vcpu, int nr_ports,
>  		goto out_rcu;
>  
>  	ret = false;
> -	if (IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode) {
> +	if (kvm_xen_has_64bit_shinfo(kvm)) {
>  		struct shared_info *shinfo = gpc->khva;
>  		pending_bits = (unsigned long *)&shinfo->evtchn_pending;
>  	} else {
> @@ -1820,7 +1820,7 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
>  	if (!kvm_gpc_check(gpc, PAGE_SIZE))
>  		goto out_rcu;
>  
> -	if (IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode) {
> +	if (kvm_xen_has_64bit_shinfo(kvm)) {
>  		struct shared_info *shinfo = gpc->khva;
>  		pending_bits = (unsigned long *)&shinfo->evtchn_pending;
>  		mask_bits = (unsigned long *)&shinfo->evtchn_mask;
> @@ -1861,7 +1861,7 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
>  			goto out_rcu;
>  		}
>  
> -		if (IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode) {
> +		if (kvm_xen_has_64bit_shinfo(kvm)) {
>  			struct vcpu_info *vcpu_info = gpc->khva;
>  			if (!test_and_set_bit(port_word_bit, &vcpu_info->evtchn_pending_sel)) {
>  				WRITE_ONCE(vcpu_info->evtchn_upcall_pending, 1);
> diff --git a/arch/x86/kvm/xen.h b/arch/x86/kvm/xen.h
> index 59e6128a7bd3..3e8c9306eb89 100644
> --- a/arch/x86/kvm/xen.h
> +++ b/arch/x86/kvm/xen.h
> @@ -248,6 +248,11 @@ struct compat_shared_info {
>  #define COMPAT_EVTCHN_2L_NR_CHANNELS (8 *				\
>  				      sizeof_field(struct compat_shared_info, \
>  						   evtchn_pending))
> +
> +/* Latched VM-wide mode; the KVM equivalent of Xen's !has_32bit_shinfo(). */
> +#define kvm_xen_has_64bit_shinfo(kvm) \
> +	(IS_ENABLED(CONFIG_64BIT) && READ_ONCE((kvm)->arch.xen.long_mode))
> +
>  struct compat_vcpu_runstate_info {
>      int state;
>      uint64_t state_entry_time;