Re: Reg. guest unaligned accesses in TCG

[Richard Henderson <richard.henderson@linaro.org>]

On 10/3/25 23:34, Pranith Kumar wrote:
> Hi Richard and Alex,
> 
> I am trying to implement efficient x86 guest on ARM64 host
> virtualization where the host has FEAT_LRCPC2 instructions. If I
> translate every x86 load to LDAPR and every x86 store to STLR, it will
> meet the memory consistency requirements.
> 
> The necessary condition for using these RCpc instructions is the
> alignment. The manual states that alignment fault will be raised if:
> 
> * Not all bytes of the memory access lie within a 16-byte quantity
> aligned to 16 bytes, and
> * The value of SCTLR_ELx.nAA applicable to the current Exception level is 0.
> 
> How does TCG check for alignment? How do I check if the guest memory
> access is within the 16-byte alignment? Will alignment fault be raised
> if it is not?

See prepare_host_addr in tcg/aarch64/tcg-target.c.inc.

a_mask is the alignment required by the guest, s_mask is the access size, so a_mask >= 
s_mask is required to be at least naturally aligned.

That's only enough to get you to the slow path for page crossings or guest misalignment. 
For additional host alignment along the fast path, you'd need to generate additional 
branches, and probably not here.

I'll warn you that I tried this once, and the extra work proving alignment overwhelmed the 
savings from substituting acquire/release for separate memory barriers.  I'd love for you 
to succeed, but this won't be a trivial task.


r~