Re: [PATCH v5 3/7] x86/fpu/xstate: Differentiate default features for host and guest FPUs

[Chao Gao]

On Fri, Apr 25, 2025 at 06:52:59AM +0800, Edgecombe, Rick P wrote:
>On Thu, 2025-04-10 at 15:24 +0800, Chao Gao wrote:
>> +
>> +	/*
>> +	 * @user_size:
>> +	 *
>> +	 * The default UABI size of the register state buffer in guest
>> +	 * FPUs. Includes all supported user features except independent
>> +	 * managed features and features which have to be requested by
>> +	 * user space before usage.
>> +	 */
>> +	unsigned int user_size;
>> +
>> +	/*
>> +	 * @features:
>> +	 *
>> +	 * The default supported features bitmap in guest FPUs. Does not
>> +	 * include independent managed features and features which have to
>> +	 * be requested by user space before usage.
>> +	 */
>> +	u64 features;
>> +
>> +	/*
>> +	 * @user_features:
>> +	 *
>> +	 * Same as @features except only user xfeatures are included.
>> +	 */
>> +	u64 user_features;
>> +};
>
>Tracing through the code, it seems that fpu_user_cfg.default_features and
>guest_default_cfg.user_features are the same, leading to
>fpu_user_cfg.default_size and guest_default_cfg.user_size being also the same.

Right. This is primarily for readability and symmetry.

I slightly prefer __guest_fpstate_reset() in this series:

	fpstate->size		= guest_default_cfg.size;
	fpstate->user_size	= guest_default_cfg.user_size;
	fpstate->xfeatures	= guest_default_cfg.features;
	fpstate->user_xfeatures	= guest_default_cfg.user_features;

over this version:

	fpstate->size		= guest_default_cfg.size;
	fpstate->xfeatures	= guest_default_cfg.features;

	/*
	 * use fpu_user_cfg for user_* settings for compatibility of exiting
	 * uAPIs.
	 */
	fpstate->user_size	= fpu_user_cfg.user_size;
	fpstate->user_xfeatures	= fpu_user_cfg.default_features;

Referencing different structures for size/xfeatures and their user_*
counterparts is not elegant to me. The need for a comment indicates that
this chunk may cause confusion. And this pattern will repeat when
initializing fpu->guest_perm in fpstate_reset().

>
>In the later patches, it doesn't seem to change the "user" parts. These
>configurations end up controlling the default size and features that gets copied
>to userspace in KVM_SET_XSAVE. I guess today there is only one default size and
>feature set for xstate copied to userspace. The suggestion from Chang was that
>it makes the code more readable, but it seems like it also breaks apart a
>unified concept for no functional benefit.

In the future, the feature and size of the uABI buffer for guest FPUs may
differ from those of non-guest FPUs. Sean rejected the idea of saving/restoring
CET_S xstate in KVM partly because:

 :Especially because another big negative is that not utilizing XSTATE bleeds into
 :KVM's ABI.  Userspace has to be told to manually save+restore MSRs instead of just
 :letting KVM_{G,S}ET_XSAVE handle the state.  And that will create a bit of a
 :snafu if Linux does gain support for SSS.

*: https://lore.kernel.org/kvm/ZM1jV3UPL0AMpVDI@xxxxxxxxxx/

[To be clear, it is not an issue caused by Chang's suggestion. v4 which adds
new members @guest_size @guest_default_features to fpu_state_config has the
same problem. i.e., fpu_user_cfg.guest_default_feaures is identical to
fpu_user_cfg.default_features, adding no functional benefit.]

>
>Maybe we don't need user_features or user_size here in vcpu_fpu_config? Or did I

I don't have a strong opinion on this. I am ok with dropping them. Do you have
a strong preference?

>get lost somewhere along the way in all the twists and turns that features and
>sizes go through.

No, your analysis is correct.

>
>