Re: [RFC PATCH] target/i386: Truncate ESP when exiting from long mode

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

On 7/26/23 01:17, Ard Biesheuvel wrote:
> While working on some EFI boot changes for Linux/x86, I noticed that TCG deviates from
> bare metal when it comes to how it handles the value of the stack pointer register RSP
> when dropping out of long mode.
> 
> On bare metal, RSP is truncated to 32 bits, even if the code that runs in 32-bit
> protected mode never uses the stack at all (and uses a long jump rather than long
> return to switch back to long mode). This means 64-bit code cannot rely on RSP
> surviving any excursions into 32-bit protected mode (with paging disabled).
> 
> Let's align TCG with this behavior, so that code that relies on RSP retaining its value
> does not inadvertently work while bare metal does not.
> 
> Observed on Intel Ice Lake cores.
> 
> Cc: Paolo Bonzini<pbonzini@redhat.com> Cc: Richard
> Henderson<richard.henderson@linaro.org> Cc: Eduardo Habkost<eduardo@habkost.net> 
> Link:https://lore.kernel.org/all/20230711091453.2543622-11-ardb@kernel.org/ 
> Signed-off-by: Ard Biesheuvel<ardb@kernel.org> --- I used this patch locally to
> reproduce an issue that was reported on Ice Lake but didn't trigger in my QEMU
> testing.
> 
> Hints welcome on where the architectural behavior is specified, and in particular,
> whether or not other 64-bit GPRs can be relied upon to preserve their full 64-bit
> length values.

No idea about chapter and verse, but it has the feel of being part and parcel with the
truncation of eip.  While esp is always special, I suspect that none of the GPRs can be 
relied on carrying all bits.

I'm happy with the change though, since similar behaviour can be observed on hw.

Acked-by: Richard Henderson <richard.henderson@linaro.org>


r~