Subject: Re: [PATCH v11 28/40] KVM: arm64: Handle SME host state when running guests

From: Catalin Marinas <[email-masked]>
Date: Wed, 23 Feb 2022 15:40:37 +0000

On Mon, Feb 07, 2022 at 03:20:57PM +0000, Mark Brown wrote:
> While we don't currently support SME in guests we do currently support it
> for the host system so we need to take care of SME's impact, including
> the floating point register state, when running guests. Simiarly to SVE
> we need to manage the traps in CPACR_RL1, what is new is the handling of
> streaming mode and ZA.
> 
> Normally we defer any handling of the floating point register state until
> the guest first uses it however if the system is in streaming mode FPSIMD
> and SVE operations may generate SME traps which we would need to distinguish
> from actual attempts by the guest to use SME. Rather than do this for the
> time being if we are in streaming mode when entering the guest we force
> the floating point state to be saved immediately and exit streaming mode,
> meaning that the guest won't generate SME traps for supported operations.
> 
> We could handle ZA in the access trap similarly to the FPSIMD/SVE state
> without the disruption caused by streaming mode but for simplicity
> handle it the same way as streaming mode for now.
> 
> This will be revisited when we support SME for guests (hopefully before SME
> hardware becomes available), for now it will only incur additional cost on
> systems with SME and even there only if streaming mode or ZA are enabled.
> 
> Signed-off-by: Mark Brown <[email-masked]>

Reviewed-by: Catalin Marinas <[email-masked]>

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