On 19.04.2021 17:57, Andrew Cooper wrote: > On 19/04/2021 16:55, Jan Beulich wrote: >> On 19.04.2021 16:45, Andrew Cooper wrote: >>> Factor out a compat boolean to remove the lfence overhead from multiple >>> is_pv_32bit_domain() calls. >>> >>> For a compat guest, the upper 32 bits of rdx are zero, so there is no need >>> to >>> have any conditional logic at all when mapping the start info page. >> Iirc the contents of the upper halves hold unspecified contents after >> a switch from compat to 64-bit mode. Therefore only with this part of >> the change dropped ... > > But we're shim, so will never ever mix compat and non-compat guests.
That's not the point: A compat guest will still cause the CPU to transition back and forth between 64-bit and compat modes. It is this transitioning which leaves the upper halves of all GPRs in undefined state (even if in reality a CPU would likely need to go through extra hoops to prevent them from being zero if they were written to in compat mode). Jan
