Regarding stacks in PSRAM: For tasks that do not call ROM code in any way (directly or indirectly), the :ref:CONFIG_FREERTOS_TASK_CREATE_ALLOW_EXT_MEM option will eliminate the check in :cpp:func:xTaskCreateStatic, allowing a task's stack to be in external RAM. However, using this is not advised.
When used at 80 MHz clock speed, external RAM must also occupy either the HSPI or VSPI bus. Select which SPI host will be used by :ref:CONFIG_SPIRAM_OCCUPY_SPI_HOST.

Chip Revisions

There are some issues with certain revisions of ESP32 that have repercussions for use with external RAM. The issues are documented in the ESP32 Series SoC Errata_ document. In particular, ESP-IDF handles the bugs mentioned in the following ways:

ESP32 Rev v0.0

ESP-IDF has no workaround for the bugs in this revision of silicon, and it cannot be used to map external PSRAM into ESP32's main memory map.

ESP32 Rev v1.0

The bugs in this revision of silicon cause issues if certain sequences of machine instructions operate on external memory. (ESP32 Series SoC Errata_ > CPU-3.2). As a workaround, the -mfix-esp32-psram-cache-issue flag has been added to the ESP32 GCC compiler such that these sequences are filtered out. As a result, the compiler only outputs code that can safely be executed. The :ref:CONFIG_SPIRAM_CACHE_WORKAROUND option can be used to enable this workaround.

Aside from linking to a recompiled version of Newlib with the additional flag, ESP-IDF also does the following:

Avoids using some ROM functions
Allocates static memory for the Wi-Fi stack

ESP32 Rev v3.0

ESP32 rev v3.0 fixes the PSRAM cache issue found in rev v1.0. When :ref:CONFIG_ESP32_REV_MIN option is set to rev v3.0, compiler workarounds related to PSRAM will be disabled. For more information about ESP32 v3.0, see ESP32 Chip Revision v3.0 User Guide_.